I want to say that I have the same problem on two different machines running the 64 bits version of Ubuntu Edgy 6.10. The problem arises both with the nv driver and the nvidia driver so I thought it should be related to something else (hopefully not a hardware bug). Anyway, there is a post on the ubuntu forums too:
Switch to console mode and back to graphic mode solves the problem until you reboot the machine. This is still very annoying. So please, if you find a solution let us know.