I personally think that a good mobile skin for XenForo would have significantly more value than an iPhone app at this point.
Not that XenForo renders badly on mobile devices, on the contrary it renders very well (
see here) but many of the UI elements are not designed for touch interaction, and therefore may be expecting hover operations or may be too small to effectively click without zooming in.
I have plans and designs for a mobile skin, though I can't say when it will debut at this point.