Index: wp-includes/class.wp-styles.php
===================================================================
--- wp-includes/class.wp-styles.php	(revision 17229)
+++ wp-includes/class.wp-styles.php	(working copy)
@@ -61,9 +61,16 @@
 		$title = isset($this->registered[$handle]->extra['title']) ? "title='" . esc_attr( $this->registered[$handle]->extra['title'] ) . "'" : '';
 
 		$end_cond = $tag = '';
-		if ( isset($this->registered[$handle]->extra['conditional']) && $this->registered[$handle]->extra['conditional'] ) {
-			$tag .= "<!--[if {$this->registered[$handle]->extra['conditional']}]>\n";
-			$end_cond = "<![endif]-->\n";
+		if ( isset($this->registered[$handle]->extra['conditional']) && $this->registered[$handle]->extra['conditional'] ) {			
+			$tag .= "<!--[if {$this->registered[$handle]->extra['conditional']}]";
+			if( strpos( $this->registered[$handle]->extra['conditional'], '!' ) === 0 ) {
+				$tag .= "-->\n";
+				$end_cond .= "<!--";
+			} else {
+				$tag .= ">\n";
+				$end_cond .= "<!";
+			}
+			$end_cond .= "[endif]-->\n";
 		}
 
 		$tag .= apply_filters( 'style_loader_tag', "<link rel='$rel' id='$handle-css' $title href='$href' type='text/css' media='$media' />\n", $handle );