AMD/ATI has released a small hotfix for their latest version of the Catalyst driver set, 8.12. This patch fixes the black screen issue affecting 4850 cards in CrossfireX mode only.
The issue does not occur if you have a single 4850 card. Here is the exact symptom this hotfix addresses:" After installing ATI Catalyst driver release 8.12 for two or more ATI Radeon HD 4850 graphics cards in a system with ATI CrossFireX technology, the system restarts and halts on a black screen. The black screen occurs prior to the loading of the desktop for the Windows Vista 64-bit or Windows Vista 32-bit operating systems."