Seems this game isn't a big fan of multi-GPU machines, trying to run the game with crossfire enabled on my PC makes the frame rate completely tank.  Thankfully the game requirements are modest so a I can still crank everything and run at 60 frames on a single 5870.  That being said if you're one of those people who buy a pair of lower spec GPU's to make up the grunt of a large single GPU you may run into some problems.

Another case of console-itis, I recently got Splinter Cell Conviction which lacked multi-GPU support as well. 

Mmmm yeah Conviction is a shame, they report they've added it in a recent patch but I didn't notice any difference.  Frame rate in conviction just seems to be un-justifiably slugish.
 
Blur is a whole different kettle of fish though, just by running Crossfire the frame rate drop's from 60 to a struggling to stay above 20 scenario (dips into single digits if you drive through a puddle).
 
Hope they sort it out, could be a real bummer for anyone rocking say a 2x4770 setup or similar.