4870: why aren’t you supported on MacPro1,1?

UPDATE: This is no longer a mystery. If you want the punchline, please proceed directly to the comments at the end of this post.

The ATI HD 4870 video card is a nice upgrade from the NVidia 8800 GT (my current card), however it’s not supported on the first generation Mac Pro (my current Mac Pro). Apple’s site states:

Compatibility:

• Requires Mac Pro (Early 2009 with 1066MHz DDR3 memory) or Mac Pro (Early 2008 with 800MHz DDR2 FB-DIMM memory) with PCI Express 2.0 slot

(The MacPro1,1 has 667 MHz DDR2 memory). I heard a rumor on the internets that this card DOES in fact work on a MacPro1,1 – with no issues or weirdness, so I had to try. My card arrived yesterday, and I’m here to report that indeed, it seems to work fine!

So far, I’ve only tried it in WoW and Motion, but so far so good… Well except for the fact that the ATI driver in 10.5.7 breaks 3d acceleration in VM guests… oh wells.

The question remains: why is this card not supported on the first-gen Mac Pro?