To everyone talking about the 2x - 4x "jump" in speed, the stats are correct. The thing is, the speed increase you get depends completely on what software you are running. If it is just opening Mail it will be a small increase, if you are actually pushing the cpu and graphics hardware like the video demo above then the performance difference will be much greater.
I can attest to this as my game currently in development has a lot of sprites and many particles on screen at once, on my iPod Touch 2G the framerate will occasionally stutter and drop a bit, on my 3G S it runs silky smooth no matter what. In a stress test of my game the 3G S can handle a lot more processor and video load at a decent frame rate than the old processor.
So the point of this post is that the speed increase depends entirely on the application and how it handles memory and utilizes the phones resources.
Oh and one last thing, for next years iPhone I highly doubt there is going to be a speed bump anywhere close to the magnitude of 3G->3G S. My bet is a redesigned case and probably a camera bump again. But it's at least a year away so who knows what will happen between now and then.