Following the newly updated Remote application from Apple, there's now a new remote application (iTunes Link) that allows you to control your XBMC Media Center with the iPhone. Obviously to use it you'll have to have a computer (or Xbox) running XBMC, which is available in versions for Xbox, Mac, Linux, and Windows. Features include:
Play, queue and shuffle the media on XBMC
View cover art for albums, TV Shows and movies
View artist and album reviews and information
'Standard' remote mode. Control XBMC with standard remote buttons
I have seen reports that it will work with the Mac-only XBMC fork Plex (though perhaps without images). It basically looks like it does for XBMC what Apple's Remote app does for the Apple TV.