Their are no workstation cards that I know of that work in SLI, unless NVIDIA came out with one.
The new SLI technology may be new, but the idea of dual graphics cards ain't. The graphics, CAD, and CGI industries have been using them for years.
Anyway, I thought I saw somewhere that the 6800GT's and Ultras had models that were SLI capable?
Anyway, it's kinda pointless to spend all that money and have all that power for a game that either doesn't utilize the power you've got, or only uses so much power and wastes the rest.
If it's running great, you can't make it run any better.