This is a copy and paste from a bug I just reported against Ubuntu Feisty.
"For months I have had 32-bit Feisty on one disk, decided to install 64-bit feisty on another.
Used the desktop CD, enabled nvidia in restricted modules manager thing before running the graphical installer.
Once installed, rebooted. Blank screen. Both screens report "No signal".
Machine does boot, and i have managed to login then install ssh "blind" and ssh into it.
Noted linux-restricted-modules-`uname -r` wasn't installed, so installed it and restarted X (all done via ssh).
Rebooted, still blank - blank from power up. No display at all.
Note: The machine is blank right from power up. I know this sounds like hardware failure, but this box has been fine for months, I find it too much of coincidence that *both* video cards should fail after an install of Feisty 64-bit. I know logic dictates that this stuff should not cause hardware failure, but the fact is it's blank from boot to X. I have removed one card (they were in SLI, now not, now only one card).
This is a core2duo based machine with twin NVidia 7900GTs, now only one. Two Viewsonic displays connected to one card via DVI. The only thing that I can think is that the nvidia blob put the cards into an odd state.
I am going to shut this down, leave the power disconnected and see if the card resets or some other magic. Other than that I have warranty on the cards and can get them replaced, but this shouldn't happen of course."
9631 is the version of the Nvidia driver shipped in Feisty.