A fella over at iPhoneDSLR.com has been working on a prototype housing for the iPhone that allows for attaching Canon EF lenses. Y'know, those big honkin' ones you see on the Canon DSLR cameras. Basically -- in theory, at least -- the iPhone lens focuses through the SLR lens, and you can manually focus and zoom the Canon lens as needed.
Definitely a neat idea, though, if you follow what he's doing, he's hitting a couple of snags. One major one being that the iPhone camera is unable to focus on the image projected by the Canon lens. Though the inventor of the housing admits he's by no means a professional photographer, I'm definitely interested in seeing if he can get the help he needs to get this to work well enough to go to market.