I change my iPhone cover like I change my pants -- often. The one I have my eye on now is this self-adhesive laser cut backing that is, wait for it, heat sensitive! That means whenever you're holding it, you'll get some SciFi-like image of your "heat aura." Colors range from black (the coldest) to red, yellow, green and blue (the hottest).
No word yet if when your iPhone is embarrassed it will turn bright blue. Buy it here for $12!
And if you're a fan of the periodic table (uh, who isn't?), you can pick your element of choice as a keychain. Kewl.