The iPhone camera lens is flush with the perfectly flat back of the device making it easy to hold a pocket magnifier loupe in front to convert the camera into a microscope. Here I am using a Radio Shack 15x 3-lens magnifier on a keycap of my Powerbook. An LED flashlight is adding light to the backlit keyboard symbols. Most any kind of loupe or pocket microscope should work. I'll be trying binoculars and spotting scopes too.
I've started a group called iPhone as microscope for anyone who wants to post interesting results.
[Original photo sent from my iPhone]
Now Wired has has this technique in their HOWTO wiki.