I was just on barefeats.com website and saw this:
2-3870s in crossfire on a mac?
Game on!
April 29th, 2008 -- ATI Radeon HD 3870 for Mac will ship the end of May. We have received confirmation from a reliable source that a ATI will definitely offer a retail version of the ATI Radeon HD 3870 with 512MB DDR4 that will boot OS X on both current and legacy Mac Pros. It will, of course, boot Windows XP or Vista in your Boot Camp partition. And, like the 2600, we expect it to support CrossFire mode under Windows if you have two installed and jumpered.
2-3870s in crossfire on a mac?
Game on!