You mean like the iPhone and Leopard?
I think those are different "beasts". Apple didn't have to think about "canibalizing" sales of the previous iPhone because there was no previous iPhone. Pre-announcing Leopard I guess did not cause much problems also. If you buy a Mac, you get the latest OS. And I guess most of the people that wanted to update old macs that didn't came with Tiger already did.
Pre announcing than in december or january or whenever the updated Mac Pro will be shipping could cause them problem of people en-mass waiting for the update. Now, that there is no official word of an update, people *are* waiting for the update, imagine if Apple did that.
Of course, there still be people that actually need a Mac Pro (and the current generation, with all it's problems and so on, it's a very good machine, IMHO), and they will buy it when they need it, no matter if next week/month/year there is a newer and better machine.
Let see if tomorrow is the day, or we must wait until January or wathever.
For the time being, I'm very happy (and productive) with my current Mac Pro (bought as soon as it arrived to Mexico, about 2 or 3 weeks after announced)
Regards,