That’s an Apple-controlled DRM issue.
The Quadro 4800 works fine with the Mac Pro… IF you buy it from Apple and it’s blessed with the magic signed code.
While there’s a story about “Oh, video cards on OSX need special super-modern EFI code support!”, it’s likely an excuse to let Apple control the hardware and therefore make a cut of the add-in card profits. Notice the Apple-blessed card costs $200 more than the exact same non-Apple-blessed card.
If I sound annoyed, it’s because I really want to run some of my cards on a Mac Pro, but can’t.
I am certain it’s not an Nvidia issue… they’d be happy to sell as many cards of as many flavors on as many platforms as they can.
Though there’s one retailer that has a big discount on the Mac version right now!