Index: wp-includes/vars.php
===================================================================
--- wp-includes/vars.php	(revision 20375)
+++ wp-includes/vars.php	(working copy)
@@ -97,3 +97,22 @@
  * @global bool $is_iis7
  */
 $is_iis7 = $is_IIS && (strpos($_SERVER['SERVER_SOFTWARE'], 'Microsoft-IIS/7.') !== false);
+
+function wp_is_mobile() {
+	static $return = null;
+
+	if ( $return !== null )
+		return $return;
+
+	if ( empty($_SERVER['HTTP_USER_AGENT']) )
+		$return = false;
+	elseif ( strpos($_SERVER['HTTP_USER_AGENT'], 'Mobile') !== false // many mobile devices (all iPhone, iPad, etc.)
+		|| strpos($_SERVER['HTTP_USER_AGENT'], 'Android') !== false
+		|| strpos($_SERVER['HTTP_USER_AGENT'], 'BlackBerry') !== false
+		|| strpos($_SERVER['HTTP_USER_AGENT'], 'Opera Mini') !== false )
+			$return = true;
+	else
+		$return = false;
+
+	return $return;
+}