I talked to a friend who works at Apple. The 8800 will not work on the Previous generation of the Mac Pro and there is no intention to provide a support path for the prior generation of mac pro systems.
The only way to get an 8800 is with the purchase of the Penryn based platform.
That is always been the case when Apple offers new slots in the machines.
The graphics card companies generally were the ones to offer the cards for the older machines with the Mac Retail editions.
But of course that has been ATI doing that for the older Mac users, can't remember many recent Nvidia retail editions offering support for the older machines with a variant of the OEM cards.