A week and a half ago, AMD released the Catalyst 8.12 package for Radeon cards. For the most part the drivers worked as planned, but post-release AMD has discovered an issue with the package that has required them to issue a hotfix, with those using the Radeon HD 4850 card in CrossfireX mode apparently being plagued by a black screen.
The hotfix only addresses this bug and only applies specifically to systems running Windows Vista with dual Radeon HD 4850s running in CrossfireX mode. If you happen to fall into that category, you can download the hotfix now and cure yourself of the issue. All other Radeon users do not have to worry about it.