Just got my 4870 from Apple (OEM kit) and would like to also confirm that the card so far works perfectly in a 2006 Mac Pro (2x 2.66Ghz Woodcrest) running 10.5.7. Had an initial scare where the machine would *bong*, but would sit at a grey screen (no Apple logo). Reseating the card fixed the problem, and I've run the card through some paces with WoW and COD4.
More importantly, the 4870 is virtually silent compared to the 3870 it replaced, which will be RMA'd very shortly for a refund. Having come initially from a passively cooled 7300GT, the 3870 was quite a shock. Not only was the fan quite loud at idle (for reference, about as loud as the optical drive spinning at max speed), it generated so much heat that it noticeably increased my HD temps in the bays above it (up to 54C), but also caused the side panel of my Mac to become very warm to the touch. Granted, I may have gotten a defective 3870, but some research on my part suggests that many others seem to be complaining of heat and fan noise with the 3870. I am now back to noise levels very close to when I had the 7300GT, but with a better performing card. The 3870 performed nicely for the price, but I think I'm sticking with my 4870 😀