There is a new hotfix for GeForce SLI customers running DirectX 9 and DirectX 10 applications: http://support.microsoft.com/kb/945149.

This hotfix resolves some scenarios where gamers have experienced lower-than-expected performance when running in SLI mode. This performance fix does not apply to all scenarios and games, but if you’ve experienced SLI performance problems with  BioShock (DX9), Company of Heroes (DX10) at high resolution, Rainbow Six Vegas, and Unreal Tournament 3 chances are it will help.

All NVIDIA-recommended hotfixes GeForce users can be found here: http://www.nvidia.com/object/windows_vista_hotfixes.html