diff -urp ./modules/system/admin.css ../drupal-HEAD-mirror/modules/system/admin.css
--- ./modules/system/admin.css	2009-05-19 12:12:41.000000000 +1200
+++ ../drupal-HEAD-mirror/modules/system/admin.css	2009-05-21 10:39:51.000000000 +1200
@@ -51,6 +51,9 @@ div.admin-requirements, div.admin-requir
   font-size: 0.9em;
   color: #444;
 }
+div.admin-theme-name-conflict {
+  color: #800;
+}
 span.admin-disabled {
   color: #800;
 }
diff -urp ./modules/system/system.admin.inc ../drupal-HEAD-mirror/modules/system/system.admin.inc
--- ./modules/system/system.admin.inc	2009-05-19 12:12:41.000000000 +1200
+++ ../drupal-HEAD-mirror/modules/system/system.admin.inc	2009-05-21 10:44:25.000000000 +1200
@@ -148,6 +148,8 @@ function system_themes_form() {
   $incompatible_core = array();
   $incompatible_php = array();
 
+  $enabled_modules = system_enabled_modules();
+  $name_conflict_modules = array();
   foreach ($themes as $theme) {
     $screenshot = NULL;
     $theme_key = $theme->name;
@@ -186,6 +188,10 @@ function system_themes_form() {
         $incompatible_php[$theme->name] = $theme->info['php'];
       }
     }
+
+    if (in_array($theme->name, $enabled_modules)) {
+      $name_conflict_modules[$theme->name] = 0;
+    }
   }
 
   $form['status'] = array(
@@ -194,6 +200,7 @@ function system_themes_form() {
     '#default_value' => $status,
     '#incompatible_themes_core' => drupal_map_assoc($incompatible_core),
     '#incompatible_themes_php' => $incompatible_php,
+    '#name_conflict_modules' => $name_conflict_modules
   );
   $form['theme_default'] = array(
     '#type' => 'radios',
@@ -230,10 +237,26 @@ function system_themes_form() {
     '#type' => 'submit',
     '#value' => t('Reset to defaults'),
   );
+  $form['#validate'][] = 'system_themes_form_validate';
 
   return $form;
 }
 
+function system_themes_form_validate($form, &$form_state) {
+  $enabled_modules = system_enabled_modules();
+
+  if (in_array($form_state['values']['theme_default'], $enabled_modules)) {
+    form_set_error($form_state['values']['theme_default'], t("The '!theme' theme could not be set as default because it conflicts with an enabled module with the same name.", array('!theme' => $form_state['values']['theme_default'])));
+  }
+  else if (is_array($form_state['values']['status'])) {
+    foreach ($form_state['values']['status'] as $key => $choice) {
+      if ($choice && in_array($key, $enabled_modules)) {
+        form_set_error($key, t("The '!theme' theme could not be enabled because it conflicts with an enabled module with the same name.", array('!theme' => $key)));
+      }
+    }
+  }
+}
+
 /**
  * Process system_themes_form form submissions.
  */
@@ -554,6 +577,36 @@ function _system_is_incompatible(&$incom
 }
 
 /**
+ * Get names of all enabled modules
+ *
+ * @return
+ *   Returns an array of the names of all enabled modules
+ */
+function system_enabled_modules() {
+  $enabled_modules = array();
+  $result = db_query("SELECT name FROM {system} WHERE type='module' AND status=1");
+  while ($module = db_fetch_object($result)) {
+    $enabled_modules[] = $module->name;
+  }
+  return $enabled_modules;
+}
+
+/**
+ * Get names of all enabled themes
+ *
+ * @return
+ *   Returns an array of the names of all enabled themes
+ */
+function system_enabled_themes() {
+  $enabled_themes = array();
+  $result = db_query("SELECT name FROM {system} WHERE type='theme' AND status=1");
+  while ($theme = db_fetch_object($result)) {
+    $enabled_themes[] = $theme->name;
+  }
+  return $enabled_themes;
+}
+
+/**
  * Menu callback; provides module enable/disable interface.
  *
  * The list of modules gets populated by module.info files, which contain each module's name,
@@ -598,6 +651,8 @@ function system_modules($form_state = ar
     return system_modules_confirm_form($files, $form_state['storage']);
   }
 
+  $enabled_themes = system_enabled_themes();
+
   $modules = array();
   $form['modules'] = array('#tree' => TRUE);
 
@@ -644,6 +699,16 @@ function system_modules($form_state = ar
         }
       }
     }
+
+    // Mark module disabled if there is an enabled module with the same name.
+    if (in_array($module->name, $enabled_themes)) {
+      $extra['theme_name_conflict'] = TRUE;
+      $extra['disabled'] = TRUE;
+    }
+    else {
+      $extra['theme_name_conflict'] = FALSE;
+    }
+
     $form['modules'][$module->info['package']][$filename] = _system_modules_build_row($module->info, $extra);
   }
   // Add basic information to the fieldsets.
@@ -667,10 +732,23 @@ function system_modules($form_state = ar
     '#value' => t('Save configuration'),
   );
   $form['#action'] = url('admin/build/modules/list/confirm');
+  $form['#validate'][] = 'system_modules_validate';
 
   return $form;
 }
 
+function system_modules_validate($form, &$form_state) {
+  $enabled_themes = system_enabled_themes();
+
+  foreach ($form_state['input']['modules'] as $group_name => $group) {
+    foreach ($group as $module_name => $module_state) {
+      if ($module_state['enable'] && in_array($module_name, $enabled_themes)) {
+        form_set_error("modules][$group_name][$module_name", t("The '!module' module could not be enabled because it conflicts with an enabled theme with the same name.", array('!module' => $module_name)));
+      }
+    }
+  }
+}
+
 /**
  * Array sorting callback; sorts modules or themes by their name.
  */
@@ -728,6 +806,12 @@ function _system_modules_build_row($info
     $status_long .= t('This module requires PHP version @php_required and is incompatible with PHP version !php_version.', array('@php_required' => $php_required, '!php_version' => phpversion()));
   }
 
+  if ($extra['theme_name_conflict']) {
+    $compatible = FALSE;
+    $status_short .= t('Name conflict with enabled theme.');
+    $status_long .= t('The name of this module conflicts with the !theme theme.', array('!theme' => $info['name']));
+  }
+
   // If this module is compatible, present a checkbox indicating
   // this module may be installed. Otherwise, show a big red X.
   if ($compatible) {
@@ -2228,6 +2312,11 @@ function theme_system_themes_form($form)
       }
       $description .= '<div class="incompatible">' . t('This theme requires PHP version @php_required and is incompatible with PHP version !php_version.', array('@php_required' => $php_required, '!php_version' => phpversion())) . '</div>';
     }
+    elseif (isset($form['status']['#name_conflict_modules'][$key])) {
+      unset($form['status'][$key]);
+      $status = theme('image', 'misc/watchdog-error.png', t('incompatible'), t('Incompatible with this version of PHP'));
+      $description .= '<div class="incompatible">' . t('This theme name conflicts with an enabled module with the same name.') . '</div>';
+    }
     else {
       $status = drupal_render($form['status'][$key]);
     }