I think WoW may only be utilizing my SLI some of the time, or else Windows is not allocating it priority. I have two GeForce GTS 250's in SLI. Some of the time, WoW runs at 60 FPS when no characters or mobs are on the screen, and then drops down to about 40 when the screen is busier. Other times, it tops out at about 45, then drops down to about 30 when the screen is busy. Sometimes, if I alt-tab out and in that seems to fix it.
I am using Windows 7 and have the latest graphics drivers installed. This problem doesn't happen in other games.