If nothing else, there's a mobile style for vBulletin that works very well over at my other site and would probably do just fine here. (In fact, I've been meaning to ask that it be installed here, so this discussion is timely.) We registered a .mobi address that takes you straight into the mobile version, which you can try out if you don't mind reading posts about motorcycles and pie: CLICKY.
An iPhone app would be great for iPhoners but would be useless for those of us who use other platforms.