I got the same problem when i switched back to the vt console or when i logged out with or without vesafb module activated.
My computer freezed in a strange 31.6Khz, 71Hz black screen mode
after i switched back (Ctrl+Alt+F1) to one of the vt consoles ... and this happened only with twinview mode or ":0.0 :0.1" display mode. In any case, it never happened during a single screen X11 configuration even with vesafb or APIC activated in the kernel.
I could get a workaround to avoid the problem but only with a two display configuration (the solution doesn't work in a twinview mode
to stop the xdm server during a current working session please use the command "/etc/init.d/xdm stop" !!! So i could return to a working console and stop manually the station after i logged back to root in order to perform an init 0.
A bit complicated, but my data are safe and the next boot didn't need to perform a "fsck" !
I continue to hope Nvidia developers would be kind enough to investigate and find a correction to that annoying bug!!
I use a Gforce4 Ti4200 video card.