There are modifications at vbulletin.org to allow you to do this but if you prefer not to have loads of mods and prefer to code different preferences your self this could be ideal
<script language=javascript> <!– if((navigator.userAgent.match(/iPhone/i)) || (navigator.userAgent.match(/iPod/i))) { location.replace(“http://www.yoursite.com/forum/index.php?styleid=6″); } –> </script>
You can get a user agent list here : http://www.user-agents.org/