Index: wp-includes/vars.php
===================================================================
--- wp-includes/vars.php	(revision 6023)
+++ wp-includes/vars.php	(working copy)
@@ -1,15 +1,24 @@
 <?php
 
 // On which page are we ?
-if ( preg_match('#([^/]+\.php)$#', $PHP_SELF, $self_matches) ) {
+if ( is_admin() ) {
+	// wp-admin pages are checked more carefully
+	preg_match('#/wp-admin/?(.*?)$#i', $PHP_SELF, $self_matches);
 	$pagenow = $self_matches[1];
-} elseif ( strpos($PHP_SELF, '?') !== false ) {
-	$pagenow = explode('/', $PHP_SELF);
-	$pagenow = trim($pagenow[(sizeof($pagenow)-1)]);
-	$pagenow = explode('?', $pagenow);
-	$pagenow = $pagenow[0];
+	$pagenow = preg_replace('#\?.*?$#', '', $pagenow);
+	if ( '' === $pagenow || 'index' === $pagenow || 'index.php' === $pagenow ) {
+		$pagenow = 'index.php';
+	} else {
+		preg_match('#(.*?)(/|$)#', $pagenow, $self_matches);
+		$pagenow = strtolower($self_matches[1]);
+		if ( '.php' !== substr($pagenow, -4, 4) )
+			$pagenow .= '.php'; // for Options +Multiviews: /wp-admin/themes/index.php (themes.php is queried)
+	}
 } else {
-	$pagenow = 'index.php';
+	if ( preg_match('#([^/]+\.php)([?/].*?)?$#i', $PHP_SELF, $self_matches) )
+		$pagenow = strtolower($self_matches[1]);
+	else
+		$pagenow = 'index.php';
 }
 
 // Simple browser detection