A few days ago I got a fancy new decal for my iPhone and I love it. It's made by a company called GelaSkins. You can even upload your own picture / logo on one.
It's made of some awesome 3M Material that makes it easy to apply and remove from the phone. The really cool part is the custom wall-paper so that when the phone is locked you see the continuity of the artwork. GelaSkins remind me a lot of Threadless (awesome t-shirts).
I might have to get an iPad one next. I like the The Great Wave