Just got back from the Apple Store in Santa Rosa where I had a chance to see the 3G S in action. It's definitely a lot faster than my regular 3G (the one I saw was using the Apple Store's very fast Wi-Fi connection, not AT&T 3G. Next time I'm there I'll do a side-by-side on 3G and see what the difference is like).
Went over to the Apple Store to try a 3G S on the AT&T network (since that's where most of my iPhone airtime is spent) but they had no iPhones hooked up for that. So off to AT&T I went.
At AT&T, they did have an iPhone 3G S connected to their network and so I was able to do a side-by-side speed test loading the Yahoo! home page. As expected, with a 50% faster processor and twice the RAM the 3G S loaded the page faster than my iPhone - but only by a few seconds (the difference in speed was nothing like the experience I had had earlier on Wi-Fi via the T1 line ($3500/month) at the Apple Store where the 3G S really flies).
On another note, AT&T told me that their 7.2Mbps download speed is already available in certain places, like Santa Rosa. It's even faster - around 18Mbps in Sacramento. No word when Sebastopol will get any of this, unfortunately.