Colorware is already doing this...
http://www.colorwarepc.com/p-103-iphone.aspx
Not an endorsement or anything, just an informational link for the topic. I have no experience with them, have just seen them on the web.
Personally, I like the idea of a hard coat of paint on the back, which is more scratch resistant, and hides scratches better than chrome or brushed aluminum.
But what I really wish Apple would do, is nix the chrome border around the face, and either make it black-chrome, or charcoal like the iPod touch.
The iPod Touch looks much more sophisticated than the iPhone, (and almost every other device, as well) just by not having the "bling" around the edge, and having it slightly different than black, but still dark.
Maybe iPhone Rev 2 will look more like the iPod touch. I would certainly buy an iPod Touch if I didn't mind carrying more than one device, and that I want cellular network access off of wi-fi. The iPhone has it's drawbacks, too.
As an digital media player with internet access, the iPod Touch is extremely hard to beat, especially with 32GB of space, and finally the inclusion of some of the iPhone's apps. That phone/cell internet functionality is the big kicker that pushes me over to the iPhone, and will probably be having me buy one at the end of the month, after the sdk announcement.