For performance we decided to compare application launch times as well as web browser performance across all three generations of iPhones. Each of the iPhones used was running an un-jailbroken version of iPhone OS 3.0 for testing.
As you can see, the iPhone 3GS is seriously faster than the original 2G. Most of this can be accounted for by the extra internet speed boost moving from Edge-only to a 3G network.
Interestingly, the iPhone 3GS is only slightly faster than the 3G. It would have been nice to see a better jump in speed here, but I suppose that will have to wait until we see improvements in the cellular network.