Index: wp-admin/themes.php
===================================================================
--- wp-admin/themes.php	(revision 11539)
+++ wp-admin/themes.php	(working copy)
@@ -109,7 +109,7 @@
 		$theme_name = is_object($theme) ? $theme->name : (is_array($theme) ? $theme['Name'] : '');
 		$details_url = add_query_arg(array('TB_iframe' => 'true', 'width' => 1024, 'height' => 800), $update['url']); //Theme browser inside WP? replace this, Also, theme preview JS will override this on the available list.
 		$update_url = wp_nonce_url('update.php?action=upgrade-theme&theme=' . urlencode($stylesheet), 'upgrade-theme_' . $stylesheet);
-		$update_onclick = 'onclick="if ( confirm(\'' . esc_js( __("Upgrading this theme will lose any customizations you have made.  'Cancel' to stop, 'OK' to upgrade.") ) . '\') ) {return true;}return false;"';
+		$update_onclick = 'onclick="if ( confirm(\'' . esc_js( __("Upgrading will remove all customizations stored within the theme’s directory.\n  'Cancel' to stop, 'OK' to upgrade.") ) . '\') ) {return true;}return false;"';
 
 		if ( ! current_user_can('update_themes') )
 			printf( '<p><strong>' . __('There is a new version of %1$s available. <a href="%2$s" class="thickbox" title="%1$s">View version %3$s Details</a>.') . '</strong></p>', $theme_name, $details_url, $update['new_version']);