Yeah, nVIDIA SLI works with Guild Wars 2. However, with GW2 and current nVIDIA driver (306.02 beta) frame rate fluctuation is worse in SLI mode.
I might be mistaken but I don't think it's a driver issue though. The general consensus around forums on the net seems.... the problem = GW2 graphics engine. They render part on cpu, part on gpu, and it keeps switching back and forth during gameplay.
You can tell when the cpu kicks in instead of gpu because I can be in the same area 5 minutes later and FPS is lower than my vsync, with no people before or after.