diff --git a/wp-includes/default-constants.php b/wp-includes/default-constants.php
index be152ea244..ff610f9902 100644
--- a/wp-includes/default-constants.php
+++ b/wp-includes/default-constants.php
@@ -258,6 +258,13 @@ function wp_default_constants( $context ) {
 			 * @since 2.1.0
 			 */
 			define('STYLESHEETPATH', get_stylesheet_directory());
+			
+			/**
+			 * Slug of the fallback theme for this install.
+			 * Will be used as the fallback if the current theme doesn't exist.
+			 * @since 3.0.0
+			 */
+			define( 'WP_FALLBACK_THEME', 'twentyten' );
 			break;
 
 	}
diff --git a/wp-includes/theme.php b/wp-includes/theme.php
index c887f99420..db4c8dfb7b 100644
--- a/wp-includes/theme.php
+++ b/wp-includes/theme.php
@@ -951,7 +951,7 @@ function get_attachment_template() {
 function get_comments_popup_template() {
 	$template = locate_template(array("comments-popup.php"));
 	if ('' == $template)
-		$template = get_theme_root() . '/default/comments-popup.php';
+		$template = get_theme_root() . '/' . WP_FALLBACK_THEME . '/comments-popup.php';
 
 	return apply_filters('comments_popup_template', $template);
 }
@@ -1160,15 +1160,13 @@ function validate_current_theme() {
 	if ( defined('WP_INSTALLING') || !apply_filters( 'validate_current_theme', true ) )
 		return true;
 
-	$fallback = 'twentyten';
-
-	if ( get_template() != $fallback && !file_exists(get_template_directory() . '/index.php') ) {
-		switch_theme($fallback, $fallback);
+	if ( get_template() != WP_FALLBACK_THEME && !file_exists(get_template_directory() . '/index.php') ) {
+		switch_theme( WP_FALLBACK_THEME, WP_FALLBACK_THEME );
 		return false;
 	}
 
-	if ( get_stylesheet() != $fallback && !file_exists(get_template_directory() . '/style.css') ) {
-		switch_theme($fallback, $fallback);
+	if ( get_stylesheet() != WP_FALLBACK_THEME && !file_exists(get_template_directory() . '/style.css') ) {
+		switch_theme( WP_FALLBACK_THEME, WP_FALLBACK_THEME );
 		return false;
 	}