While this should be taken as a pinch of salt, Intel's upcoming P45 chipset apparently has some CrossFire issues when using Catalyst 8.4.

It is known that the P45 chipset is almost ready for shipment. So far, some issues have surfaced during tests. CrossFire does not work very well with Catalyst 8.4 drivers. The screen will be messed up. It could be a BIOS issue or driver issue that needs to be resolved by both Intel and ATI.