Nvidia recently released a new hotfix for GeForce SLI customers running DirectX9 and Direct 10 applications.

This hotfix resolves some scenarios in which the performance of games can be lower than expected when running in SLI mode. This performance fix does not apply to all scenarios and games but there are some games that show some good performance gains such as: BioShock DX9, COH DX10 at high res, Rainbow Six Vegas, UT3 demo and so on.

All hotfixes can be found here: http://www.nvidia.com/object/windows_vista_hotfixes.html.