It should be automatic, but if your cookies have set you to view the full version you'll need to clear your cookie cache. Problem is that logs you out of everything that's cookied prior. I wish you could chose which cookies to clear out.
In iOS 5 you can clear specific cookies. I've seen a lot of complaints on IPB about the mobile version being wonky in switching back and forth, so I'm betting they release a fix for this soon.
On the mobile template, click on the bock with four squares in the top right, then click on sign in. It will take you to a new screen for log in. I will try and test on a friends EVO if I can.