I've done several side-by-side comparisons with a coworker's 3GS vs. my 3G and I do not find the 3GS speed improvements significantly noticeable for *most* things. Yes, very processor intensive tasks are certainly quicker, as is browsing on a WiFi connection (which I use my macbook for). But as soon as the connection speeds drop to 3G, the network seems to be the bottleneck rather than the phone based on most of the websites I went to in a side-by-side comparison. While the 3GS certainly boots up much quicker, I did not notice much of a speed difference in launching/using most of the iPhone's apps.
I'm not eligible for an upgrade until December, and I don't see a reason to take my upgrade then just to screw myself out of the next model that follows the 3GS. It's just not a big enough upgrade for me.