I've skimmed my way through this thread reading most (but not all) of the entries. Haven't got anything of particular interest to add other than to chuck in my two pence worth!
I finally gave in and bought a Mac Pro 2.66GHz, 3GB ram, 250GB boot, 1TB data in April. I'd been waiting since Autumn 2006 for the upgrade that everyone is still waiting for now (apart from the 8-core addition which is not what most people - me included - we really waiting for). I still don't know why I left it so long! My Mac Mini G4 was a nice machine but the Pro is just so stonkingly good and likely to remain so whatever updates are on the horizon.
I agree with the points made earlier. What difference will the odd percent improvement here and there really mean? Better video card? Maybe, but again what difference will it really make? Unless you'll be doing something that will push the current Pro configuration to its limit or you just hate the case then I see no reason to wait - even if they release an update next week! The current machine will run Leopard and whatever comes next (OS X Tabby? OS X Ginger Tom?) with no problem right up to and probably beyond the London Olympics...