The iPhone 4 has been released. Perhaps you bought one (or will buy one). Perhaps not. But if you did, you should play games on it. No, really. You should.
• Enigmo 2: Cleaner graphics • FarmVille: Better touch controls • Real Racing: Spruced up graphics, improved frame rate, iPhone 4 motion support • Tiger Woods PGA Tour: Doesn't have full iPhone 4 motion support, but better graphics
The first "exclusive" iPhone 4 dedicated game is Eliminate: Gun Range. The game offers modern weapons for players to test out on a virtual firing range. It takes advantage of the iPhone 4's gyroscope. 3G support is forthcoming, but it is currently iPhone 4 only.
It's likely that most popular iPhone 3GS games will get iPhone 4 support through updates.