Seems to me that after every update, people start saying they're waiting for the next one, because some new CPU or GPU is on the horizon. In the Core Duo days it was Merom, now it's Nehelem.
The bottom line is this: The current machines are screamers, and some of the most affordably/reasonably priced Macs ever. If this were July, I'd say to purchase now.
However, I expect an update in the first quarter of 2010. Can you wait up to four more months? If so, it's a good idea.
I always wait, and only buy immediately after an update. But I don't get hung up on when the "next" thing is coming. There's always a next thing.