If you want to serve up something special to your mobile visitors you can use this snippet to find out when they have arrived on your website. To achieve this you have to visit detectmobilebrowsers.mobi and upload it to your theme directory. Place at the top of your header file. Make sure you edit line 5 of this to your own theme.

$mobile = mobile_device_detect(); 
if ($mobile==true) { 
header( 'Location: http://your-website.com/?theme=Your_Mobile_Theme' ) ;}