Index: wp-includes/template-loader.php
===================================================================
--- wp-includes/template-loader.php	(revision 12294)
+++ wp-includes/template-loader.php	(working copy)
@@ -3,6 +3,7 @@
  * Loads the correct template based on the visitor's url
  * @package WordPress
  */
+$template = null;
 if ( defined('WP_USE_THEMES') && constant('WP_USE_THEMES') ) {
 	do_action('template_redirect');
 	if ( is_robots() ) {
@@ -12,54 +13,24 @@
 		do_feed();
 		return;
 	} else if ( is_trackback() ) {
-		include(ABSPATH . 'wp-trackback.php');
-		return;
+		$template = ABSPATH . 'wp-trackback.php';
 	} else if ( is_404() && $template = get_404_template() ) {
-		include($template);
-		return;
 	} else if ( is_search() && $template = get_search_template() ) {
-		include($template);
-		return;
 	} else if ( is_tax() && $template = get_taxonomy_template()) {
-		include($template);
-		return;
 	} else if ( is_home() && $template = get_home_template() ) {
-		include($template);
-		return;
 	} else if ( is_attachment() && $template = get_attachment_template() ) {
 		remove_filter('the_content', 'prepend_attachment');
-		include($template);
-		return;
 	} else if ( is_single() && $template = get_single_template() ) {
-		include($template);
-		return;
 	} else if ( is_page() && $template = get_page_template() ) {
-		include($template);
-		return;
 	} else if ( is_category() && $template = get_category_template()) {
-		include($template);
-		return;
 	} else if ( is_tag() && $template = get_tag_template()) {
-		include($template);
-		return;
 	} else if ( is_author() && $template = get_author_template() ) {
-		include($template);
-		return;
 	} else if ( is_date() && $template = get_date_template() ) {
-		include($template);
-		return;
 	} else if ( is_archive() && $template = get_archive_template() ) {
-		include($template);
-		return;
 	} else if ( is_comments_popup() && $template = get_comments_popup_template() ) {
-		include($template);
-		return;
 	} else if ( is_paged() && $template = get_paged_template() ) {
-		include($template);
-		return;
 	} else if ( file_exists(TEMPLATEPATH . "/index.php") ) {
-		include(TEMPLATEPATH . "/index.php");
-		return;
+		$template = TEMPLATEPATH . "/index.php";
 	}
 } else {
 	// Process feeds and trackbacks even if not using themes.
@@ -70,9 +41,11 @@
 		do_feed();
 		return;
 	} else if ( is_trackback() ) {
-		include(ABSPATH . 'wp-trackback.php');
-		return;
+		$template = ABSPATH . 'wp-trackback.php';
 	}
 }
 
-?>
\ No newline at end of file
+if ( $template = apply_filters('template_include', $template) )
+	include($template);
+
+?>
Index: wp-includes/theme.php
===================================================================
--- wp-includes/theme.php	(revision 12294)
+++ wp-includes/theme.php	(working copy)
@@ -991,7 +991,8 @@
 	if ( is_array($wp_query->query_vars) )
 		extract($wp_query->query_vars, EXTR_SKIP);
 
-	require_once($_template_file);
+	if ( $_template_file = apply_filters('template_include',$_template_file) )
+            require_once $_template_file;
 }
 
 /**