Fore those who have adopted their iPhones as their primary snapshot cameras have realized that the digital zoom gets them only so close and kind of tanks photo quality in the process. What you really need is a telephoto lens, and that's where this kit comes in, from photography tips-and-tricks site Photojojo.
This $35 telephoto lens is not unlike other attachable lenses for smartphones. But, while those are magnetic and work on any mobile device, this one requires putting on an included case, so it's only for iPhone 3/3GS and 4 models. With the case on, you just screw the lens onto the case and you're ready to shoot. Just twist the barrel until your subject's in focus and start snapping.
Photojojo has some sample photos up and they look pretty good. There is some vignetting, but that's easily cropped out if you don't want it for low-fi aesthetic. It looks like it might be a bit awkward to use in the hand, too, just because it'll add some unbalanced weight. The kit includes a tabletop tripod and a clamp to hold the iPhone in place, though, so you don't always have to hold it.