Wonder which iPhone you're holding in your hand? A new Apple support note points to the differences between the iPhone, 3G and 3GS models. And don't forget the iPhone 3GS China.
The first iPhone has a metal case, so it's easy to pick out. However, the iPhone 3G and 3GS appear very similar on first look.
You can distinguish iPhone 3GS from iPhone by its plastic case. Additionally, the serial number is no longer on the back case imprint. You'll find the serial number for the iPhone 3GS on the SIM tray. You can distinguish iPhone 3GS from iPhone 3G by the imprint on the back of the phone. The entire imprint on the back of iPhone 3GS is bright silver (not just the Apple logo).
I don't get why all that verbiage is so prominent on the back of the iPhone 3GS. It looks terrible. The older 3G has a much cleaner look. Since most users put the iPhone in a skin or rubber case, the fine print on the 3GS isn't much of a problem, I guess.
Here's the support doc for the technical specs for each model.