For many who had the combination of a pair of Radeon HD4850 in Crossfire Mode and Microsoft's Vista these new drivers meant black screens at boot and BSODs if they got into the OS.
ATi has now released a patch to fix all of that though.
Read more here.
ATI programmers have released a hotfix that corrects the black screen and boot OS failure when Catalyst 8.12 is installed on Windows Vista, and two or more ATI Radeon HD 4850 video cards are working in CrossFire X mode. This issue only occurs when two or more ATI Radeon HD 4850 graphic cards are installed in the system with ATI CrossFireX technology. One single ATI Radeon HD 4850 card will not encounter this issue.