I think this is probably not an Ubuntu issue because there is a similar thread on the arch linux forums:
atari is your problem with the 64-bit driver?
I am running arch linux x86_64 with the nvidia-256.53-1-x86_64 driver on a GeForce 8400 GS card. When I start X, my monitor reports "Cable disconnected" When I look at the terminal output it appears that x has started and gnome is running. there are no xorg errors.
I can use the nouveau driver. On the arch forums, some users report that they can fix the issue by rolling back to the previous driver.