I am really shocked by some of the comments in this thread. This thread is not promoting anything. It's not selling iPhone and it's not selling vBulletin.
As for the iPhone, I do not regret my purchase of either the 3G, 3GS, or iPhone 4 for a second. When the iPhone entered the market, most smartphones had a Godawful interface, required an easy-to-lose stylus, and barely had WAP2 web browser support . The main choices were Blackberry, Windows Mobile 6, Palm OS, and Symbian.
The fact that NOW, there are devices like Android and Palm Pre which have learned many lessons from Apple, means that I think iPhone deserves a little more respect than the mud being slung at it. If you don't personally like the iPhone, that's perfectly fine. But the mobile phone and smartphone market would not be nearly as evolved (and probably stylus or handwriting-recognition based) if it weren't for the iPhone.
Using vBulletin on a mobile web browser, is not an enjoyable experience. XenForo is much easier to use on mobile web browsers, but there are still some issues especially with the editor, and it would benefit greatly from using the mobile paradigms.
I'm not so blinded with rage towards vBulletin that I can't see when they do something right, and looking at the video demonstration (narrated with a deep British accent, hmm!), I am impressed by the vBulletin iPhone app. They clearly designed it for someone on-the-go who doesn't have time for 100kb of javascript and crap to load that they're not even going to use. From what I can see, I think it would be very popular for those still using vBulletin.
I think it is crucial that XenForo, at the very least, offer a mobile-optimized style. I also hope they will consider an iPhone app followed by an Android app.