I'm running 2x6850s and I'm getting very good scaling. I'm usually averaging around 60fps on a 1680x1050 monitor with all setting on high except shadows and Post-Processing on medium.
Everything runs rather smoothly, and I'm happy with the performance. The only strange thing that happens is that when I open up the world map and then close it, the framerate drops by about 20, as if it disables the second card. I have to Alt-Tab out and then back in for the frame rates to bounce back up.
Anyone else experiencing this?