I can get both cards installed and running on the latest drivers with no problem though. Just can't CrossFire them. I contacted ATI and they had this to say:
You can run CF within the major series so a 5870 + 5850 and 5770 + 5750. We do not currently support 5800 series + 5700 series.
Sad to hear since the GPU cores are nearly identical. The Lucid HYDRA 200 is only a couple weeks away from coming out if all goes well!