diff -urp ./modules/system/admin.css ../DRUPAL-6-CHANGES-NOGIT/modules/system/admin.css
--- ./modules/system/admin.css	2009-05-18 15:49:42.000000000 +1200
+++ ../DRUPAL-6-CHANGES-NOGIT/modules/system/admin.css	2009-05-18 16:10:13.000000000 +1200
@@ -51,6 +51,9 @@ div.admin-dependencies, div.admin-requir
   font-size: 0.9em;
   color: #444;
 }
+div.admin-theme-name-conflict {
+  color: #f00;
+}
 span.admin-disabled {
   color: #800;
 }
diff -urp ./modules/system/system.admin.inc ../DRUPAL-6-CHANGES-NOGIT/modules/system/system.admin.inc
--- ./modules/system/system.admin.inc	2009-05-18 15:49:42.000000000 +1200
+++ ../DRUPAL-6-CHANGES-NOGIT/modules/system/system.admin.inc	2009-05-18 16:10:13.000000000 +1200
@@ -196,6 +196,8 @@ 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;
@@ -234,6 +236,10 @@ 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(
@@ -242,6 +248,7 @@ 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',
@@ -256,9 +263,25 @@ 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 a module with the same name is currently enabled.", 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 a module with the same name is currently enabled.", array('!theme' => $key)));
+      }
+    }
+  }
+}
+
 /**
  * Process system_themes_form form submissions.
  */
@@ -598,6 +621,36 @@ function _system_is_incompatible(&$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 $enabled_themes;
+}
+
+/**
  * Menu callback; provides module enable/disable interface.
  *
  * Modules can be enabled or disabled and set for throttling if the throttle module is enabled.
@@ -632,6 +685,8 @@ function system_modules($form_state = ar
   }
   $dependencies = array();
 
+  $enabled_themes = system_enabled_themes();
+
   // Store module list for validation callback.
   $form['validation_modules'] = array('#type' => 'value', '#value' => $files);
 
@@ -673,7 +728,7 @@ function system_modules($form_state = ar
     if ($file->throttle) {
       $throttle[] = $file->name;
     }
-
+    
     $dependencies = array();
     // Check for missing dependencies.
     if (is_array($file->info['dependencies'])) {
@@ -703,6 +758,7 @@ function system_modules($form_state = ar
       }
     }
 
+    $disabled_by_dependents = FALSE;
     // Mark dependents disabled so user can not remove modules being depended on.
     $dependents = array();
     foreach ($file->info['dependents'] as $dependent) {
@@ -710,12 +766,25 @@ function system_modules($form_state = ar
         $dependents[] = t('@module (<span class="admin-enabled">enabled</span>)', array('@module' => $files[$dependent]->info['name']));
         $disabled[] = $filename;
         $form['disabled_modules']['#value'][$filename] = TRUE;
+        $disabled_by_dependents = TRUE;
       }
       else {
         $dependents[] = t('@module (<span class="admin-disabled">disabled</span>)', array('@module' => $files[$dependent]->info['name']));
       }
     }
 
+    // Mark module disabled if there is an enabled module with the same name.
+    if (in_array($file->name, $enabled_themes)) {
+      $form['description'][$filename]['theme_conflict'] = array(
+          '#value' => t('Conflicts with an enabled theme with the same name'),
+          '#prefix' => '<div class="admin-theme-name-conflict">',
+          '#suffix' => '</div>',
+      );
+      if (!$disabled_by_dependents) {  // check it hasn't already been disabled
+        $disabled[] = $filename;
+      }
+    }
+
     // Add text for enabled dependents.
     if (!empty($dependents)) {
       $form['description'][$filename]['required'] = array(
@@ -768,10 +837,23 @@ 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) {
+  $enabled_themes = system_enabled_themes();
+
+  if (is_array($form_state['values']['status'])) {
+    foreach ($form_state['values']['status'] as $key => $choice) {
+      if ($choice && in_array($key, $enabled_themes)) {
+        form_set_error($key, t("The '!module' module could not be enabled because a theme with the same name is currently enabled.", array('!module' => $key)));
+      }
+    }
+  }
+}
+
 /**
  * Array sorting callback; sorts modules or themes by their name.
  */
@@ -2184,6 +2266,11 @@ function theme_system_themes_form($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('name conflict'), t('Name conflict with enabled module with same name'));
+      $description .= '<div class="incompatible">'. t('This theme name conflicts with an enabled module with the same name.', array('!core_version' => VERSION)) .'</div>';
+    }
     else {
       $status = drupal_render($form['status'][$key]);
     }