Index: modules/block/block.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/block/block.module,v
retrieving revision 1.245
diff -u -r1.245 block.module
--- modules/block/block.module	18 Dec 2006 21:49:39 -0000	1.245
+++ modules/block/block.module	30 Dec 2006 15:36:17 -0000
@@ -710,3 +710,16 @@
   }
   return $blocks[$region];
 }
+
+/**
+ * Implementation of hook_system_notify()
+ *
+ * Ensure that blocks are in place for themes being enabled.
+ */
+function block_system_notify($type, $disabled, $enabled, $new = array()) {
+  if ($type == 'theme') {
+    foreach($enabled as $theme) {
+      system_initialize_theme_blocks($theme);
+    }
+  }
+}
Index: modules/system/system.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.module,v
retrieving revision 1.433
diff -u -r1.433 system.module
--- modules/system/system.module	27 Dec 2006 21:59:32 -0000	1.433
+++ modules/system/system.module	30 Dec 2006 15:36:19 -0000
@@ -1188,6 +1188,13 @@
 
 function system_themes_submit($form_id, $form_values) {
 
+  // Store list of previously enabled themes and disable all themes
+  $wasenabled = $isenabled = array();
+  foreach (list_themes() as $theme) {
+    if ($theme->status) {
+      $wasenabled[] = $theme->name;
+    }
+  }
   db_query("UPDATE {system} SET status = 0 WHERE type = 'theme'");
 
   if ($form_values['op'] == t('Save configuration')) {
@@ -1195,10 +1202,7 @@
       foreach ($form_values['status'] as $key => $choice) {
         // Always enable the default theme, despite its status checkbox being checked:
         if ($choice || $form_values['theme_default'] == $key) {
-          // If theme status is being set to 1 from 0, initialize block data for this theme if necessary.
-          if (db_num_rows(db_query("SELECT status FROM {system} WHERE type = 'theme' AND name = '%s' AND status = 0", $key))) {
-            system_initialize_theme_blocks($key);
-          }
+          $isenabled[] = $key;
           db_query("UPDATE {system} SET status = 1 WHERE type = 'theme' and name = '%s'", $key);
         }
       }
@@ -1215,8 +1219,12 @@
   else {
     variable_del('theme_default');
     db_query("UPDATE {system} SET status = 1 WHERE type = 'theme' AND name = 'garland'");
+    $isenabled = array('garland');
   }
 
+  // Notify interested parties about theme related changes
+  module_invoke_all('system_notify', 'theme', array_diff($wasenabled, $isenabled), array_diff($isenabled, $wasenabled));
+
   menu_rebuild();
   drupal_set_message(t('The configuration options have been saved.'));
   return 'admin/build/themes';
@@ -1474,6 +1482,8 @@
     }
   }
 
+  $old_module_list = module_list();
+
   if (!empty($enable_modules)) {
     module_enable($enable_modules);
   }
@@ -1481,8 +1491,6 @@
     module_disable($disable_modules);
   }
 
-  $old_module_list = module_list();
-
   // Install new modules.
   foreach ($new_modules as $key => $module) {
     if (!drupal_check_module($module)) {
@@ -1490,6 +1498,10 @@
     }
   }
   drupal_install_modules($new_modules);
+  
+  // Notify interested parties about module related changes
+  $oldmodules = array_keys($old_module_list);
+  module_invoke_all('system_notify', 'module', array_diff($oldmodules, $enable_modules), array_diff($enable_modules, $oldmodules), $new_modules);
 
   $current_module_list = module_list(TRUE, FALSE);