The guys at AnandTect have run some nice tests comparing the iPhone 3GS, the iPhone 3G, and the Palm Pre. These tests include web loading times as well as application launch times - and come out with a pretty favorable result for the 3GS.
The new 3GS renders web pages 128% faster, on average, than the old iPhone 3G. The 45% clock speed boost alone isn't enough to generate such a large performance increase, this is a new microarchitecture. Also, note that the 3GS' performance mimics that of the Palm Pre - another Cortex A8 based phone.