Flickr user curiouslee noticed that the iPhone's camera lens is flush to the back of the device, and took a photo with a pocket magnifying glass held against it. The results looks good (that's the command key on his Powerbook). He's started a Flickr group for anyone else who wants to give the idea a try (this one looks pretty good).
While you're at it, here's a good article on taking macro photos with a cell phone.
[Via Make Blog]