I saw a post on Jennifer McGuire's blog a while back about this website where you can create a personalized photo iPhone case. How cool is that?!
The company is called Uncommon and their website is getuncommon.com.
These are screenshots of the cases I designed for the iPhone 4, but you can also create cases for the 3G and 3GS for $5 more. If you don't want a photo case, you can choose from their designs, which are also really fun. Check them out!