Flickr-er Daniel Forsythe used his knowledge of cellphone camera lenses to make this quick-and-dirty manual focus hack for the iPhone, eliminating the need of tacky plastic addons to mod the iPhone's camera. It's simple. All you have to do is crack open the glue surrounding the lens. The lenses used on both the 3G and the original iPhone can be turned to focus them -- usually the focus is set to infinity at the factory (the small sensor size takes care of depth-of-field) but you can change it to a close-focussing macro lens. As Daniel points out, this makes the camera much more useful for shots of barcodes or other note-taking.
Some iPhone disassembly is required, and Daniel has full instructions on his site. The whole thing is reversible, though, so if you're happy about voiding your waranty, things shouldn't go too badly wrong.