This seems highly unlikely. Why would Apple start manufacturing computers for which they have no software? It doesn't make a lot of sense. I'm obviously not an expert, but I'd reckon that Apple develops the hardware and software concurrently.
The quad core iMacs have almost undoubtedly been in production for around a month now and I'd be seriously surprised if they're shipping with anything other than 10.6.1.
Well we'll see. They did wait to ship the Magic Mouse without the iMac until they had a software update.
Also, I do think they have drivers made. More like they are holding until they are sure there are no driver issues that may impact performance.