Index: includes/common.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/common.inc,v
retrieving revision 1.898
diff -u -p -r1.898 common.inc
--- includes/common.inc	12 May 2009 08:37:44 -0000	1.898
+++ includes/common.inc	12 May 2009 13:09:09 -0000
@@ -3232,8 +3232,7 @@ function drupal_alter($type, &$data) {
 function drupal_get_page($content = NULL) {
   // Initialize page array with defaults. @see hook_elements() - 'page' element.
   $page = element_info('page');
-  $page['content'] = is_array($content) ? $content : array('main' => array('#markup' => $content));
-
+  system_set_content_block($content);
   return $page;
 }
 
Index: modules/block/block.admin.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/block/block.admin.inc,v
retrieving revision 1.37
diff -u -p -r1.37 block.admin.inc
--- modules/block/block.admin.inc	7 May 2009 15:29:08 -0000	1.37
+++ modules/block/block.admin.inc	12 May 2009 13:09:09 -0000
@@ -36,6 +36,9 @@ function block_admin_display_form(&$form
 
   $block_regions = system_region_list($theme_key) + array(BLOCK_REGION_NONE => '<' . t('none') . '>');
 
+  // Use form submission data to show values if showing the form again.
+  $blocks = (!empty($form_state['values']) ? $form_state['values'] : $blocks);
+  
   // Weights range from -delta to +delta, so delta should be at least half
   // of the amount of blocks present. This makes sure all blocks in the same
   // region get an unique weight.
@@ -95,6 +98,19 @@ function block_admin_display_form(&$form
 }
 
 /**
+ * Validate main blocks administration form submissions.
+ */
+function block_admin_display_form_validate(&$form, &$form_state) {
+  foreach ($form_state['values'] as $key => $block) {
+    if (($block['module'] == 'system') && ($block['delta'] == 'main') && ($block['region'] == BLOCK_REGION_NONE)) {
+      form_set_error($key, t('Disabling the main page content block makes your system unusable. You should always enable it to display in a region.'));
+      // Suggest to put it back into the content region.
+      form_set_value($form[$key]['region'], 'content', $form_state);
+    }
+  }
+}
+
+/**
  * Process main blocks administration form submissions.
  */
 function block_admin_display_form_submit($form, &$form_state) {
Index: modules/block/block.install
===================================================================
RCS file: /cvs/drupal/drupal/modules/block/block.install,v
retrieving revision 1.22
diff -u -p -r1.22 block.install
--- modules/block/block.install	6 May 2009 10:39:42 -0000	1.22
+++ modules/block/block.install	12 May 2009 13:09:09 -0000
@@ -178,6 +178,14 @@ function block_schema() {
  */
 function block_install() {
   drupal_install_schema('block');
+  
+  // Block should go first so that other modules can alter its output during hook_page_alter().
+  db_update('system')
+    ->fields(array(
+      'weight' => -5,
+    ))
+    ->condition('name', 'block')
+    ->execute();
 }
 
 /**
@@ -186,3 +194,12 @@ function block_install() {
 function block_uninstall() {
   drupal_uninstall_schema('block');
 }
+
+/**
+ * Set system.weight for easy use during hook_page_alter().
+ */
+function block_update_7000() {
+  $ret = array();
+  $ret[] = update_sql("UPDATE {system} SET weight = -5 WHERE name = 'block'");
+  return $ret;
+}
Index: modules/block/block.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/block/block.module,v
retrieving revision 1.330
diff -u -p -r1.330 block.module
--- modules/block/block.module	7 May 2009 15:29:08 -0000	1.330
+++ modules/block/block.module	12 May 2009 13:09:09 -0000
@@ -760,6 +760,11 @@ function template_preprocess_block(&$var
   $variables['block_zebra'] = ($block_counter[$variables['block']->region] % 2) ? 'odd' : 'even';
   $variables['block_id'] = $block_counter[$variables['block']->region]++;
 
+  if (is_array($variables['block']->content)) {
+    // Render the block contents if it is not already rendered.
+    $variables['block']->content = drupal_render($variables['block']->content);
+  }
+
   $variables['template_files'][] = 'block-' . $variables['block']->region;
   $variables['template_files'][] = 'block-' . $variables['block']->module;
   $variables['template_files'][] = 'block-' . $variables['block']->module . '-' . $variables['block']->delta;
Index: modules/system/system.admin.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.admin.inc,v
retrieving revision 1.142
diff -u -p -r1.142 system.admin.inc
--- modules/system/system.admin.inc	12 May 2009 08:37:45 -0000	1.142
+++ modules/system/system.admin.inc	12 May 2009 13:09:10 -0000
@@ -179,7 +179,9 @@ function system_themes_form() {
     }
     else {
       // Ensure this theme is compatible with this version of core.
-      if (!isset($theme->info['core']) || $theme->info['core'] != DRUPAL_CORE_COMPATIBILITY) {
+      // Require the 'content' region to make sure the main page 
+      // content has a common place in all themes.
+      if (!isset($theme->info['core']) || ($theme->info['core'] != DRUPAL_CORE_COMPATIBILITY) || (!isset($theme->info['regions']['content']))) {
         $incompatible_core[] = $theme->name;
       }
       if (version_compare(phpversion(), $theme->info['php']) < 0) {
Index: modules/system/system.install
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.install,v
retrieving revision 1.322
diff -u -p -r1.322 system.install
--- modules/system/system.install	12 May 2009 08:31:17 -0000	1.322
+++ modules/system/system.install	12 May 2009 13:09:10 -0000
@@ -3316,7 +3316,7 @@ function system_update_7020() {
 }
 
 /**
- * Add help block to the help region, migrate custom variables to blocks.
+ * Add new blocks to new regions, migrate custom variables to blocks.
  */
 function system_update_7021() {
   $ret = array();
@@ -3328,6 +3328,8 @@ function system_update_7021() {
     $themes_with_blocks[] = $theme->name;
     // Add new system generated help block.
     $ret[] = update_sql("INSERT INTO {block} (module, delta, theme, status, weight, region, pages, cache) VALUES ('system', 'help', '" . $theme->name . "', 1, 0, 'help', '', 1)");
+    // Add new system generated main page content block.
+    $ret[] = update_sql("INSERT INTO {block} (module, delta, theme, status, weight, region, pages, cache) VALUES ('system', 'main', '" . $theme->name . "', 1, 0, 'content', '', -1)");
   }
 
   // Migrate contact form information.
Index: modules/system/system.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.module,v
retrieving revision 1.692
diff -u -p -r1.692 system.module
--- modules/system/system.module	12 May 2009 08:37:45 -0000	1.692
+++ modules/system/system.module	12 May 2009 13:09:10 -0000
@@ -889,6 +889,11 @@ function system_user_timezone(&$edit, &$
  * Implementation of hook_block_list().
  */
 function system_block_list() {
+  $blocks['main'] = array(
+    'info' => t('Main page content'),
+     // Cached elsewhere.
+    'cache' => BLOCK_NO_CACHE,
+  );
   $blocks['powered-by'] = array(
     'info' => t('Powered by Drupal'),
     'weight' => '10',
@@ -957,6 +962,10 @@ function system_block_save($delta = '', 
 function system_block_view($delta = '') {
   $block = array();
   switch ($delta) {
+    case 'main':
+      $block['subject'] = NULL;
+      $block['content'] = system_set_content_block();
+      return $block;
     case 'powered-by':
       $image_path = 'misc/' . variable_get('drupal_badge_color', 'powered-blue') . '-' . variable_get('drupal_badge_size', '80x15') . '.png';
       $block['subject'] = NULL;
@@ -979,6 +988,23 @@ function system_block_view($delta = '') 
 }
 
 /**
+ * Memory for the page content so that it can be put into a region as a block.
+ *
+ * Given the nature of the Drupal page handling, this will be called once with
+ * a string or array. We store that and return it later as the block is being
+ * displayed.
+ */
+function system_set_content_block($content = NULL) {
+  $content_block = &drupal_static(__FUNCTION__, NULL);
+  if (!empty($content)) {
+    $content_block = (is_array($content) ? $content : array('main' => array('#markup' => $content)));
+  }
+  else {
+    return $content_block;
+  }
+}
+
+/**
  * Provide a single block on the administration overview page.
  *
  * @param $item
Index: profiles/default/default.profile
===================================================================
RCS file: /cvs/drupal/drupal/profiles/default/default.profile,v
retrieving revision 1.41
diff -u -p -r1.41 default.profile
--- profiles/default/default.profile	30 Apr 2009 21:44:20 -0000	1.41
+++ profiles/default/default.profile	12 May 2009 13:09:10 -0000
@@ -91,9 +91,19 @@ function default_profile_task_list() {
  */
 function default_profile_tasks(&$task, $url) {
   
-  // Enable 5 standard blocks.
+  // Enable some standard blocks.
   $values = array(
     array(
+      'module' => 'system',
+      'delta' => 'main',
+      'theme' => 'garland',
+      'status' => 1,
+      'weight' => 0,
+      'region' => 'content',
+      'pages' => '',
+      'cache' => -1,
+    ),
+    array(
       'module' => 'user',
       'delta' => 'login',
       'theme' => 'garland',
Index: profiles/expert/expert.profile
===================================================================
RCS file: /cvs/drupal/drupal/profiles/expert/expert.profile,v
retrieving revision 1.7
diff -u -p -r1.7 expert.profile
--- profiles/expert/expert.profile	30 Apr 2009 21:44:20 -0000	1.7
+++ profiles/expert/expert.profile	12 May 2009 13:09:10 -0000
@@ -42,9 +42,20 @@ function expert_profile_task_list() {
  * Perform any final installation tasks for this profile.
  */
 function expert_profile_tasks(&$task, $url) {
-  // Enable 4 standard blocks.
+
+  // Enable some standard blocks.
   $values = array(
     array(
+      'module' => 'system',
+      'delta' => 'main',
+      'theme' => 'garland',
+      'status' => 1,
+      'weight' => 0,
+      'region' => 'content',
+      'pages' => '',
+      'cache' => -1,
+    ),
+    array(
       'module' => 'user',
       'delta' => 'login',
       'theme' => 'garland',