I have the same issue. I play WoW and there are times I can play for hours with no problems but then I can have this blank screen issue show up several times within a few minutes and then gone again. I'm sure we're not using the same monitor or computer and I have been keeping my Nvidia drivers up-to-date. Have anyone found out what the cause is?
Dell XPS 700
2 X Gforce GTX 275 (non-SLi mode)
Dell 17" Monitor
Windows 7 64-bit OS