Using Safari gives you the mobile version?
Anyway, vB4 had a mobile version available and you may set it up so that users using a mobile device will automatically view the site using the mobile style. You may test/see it on vbulletin.com/forums
vB5 uses a responsive style which will be good for browsers and mobile devices. You may test/see it on
http://www.vbulletin.com/vb5demo/