AFAIK the nVidia guys are a touch sparse on the forum in the last couple months. I'm sure they're keeping an eye on it but don't seem to be active at present. (Andy Mecham is missed)
I have experienced this issue with screen blanking while the system appears to be operating otherwise. This has happened on exit of an Xserver (4.3.0, servers set to restart when exiting) and on failed load of Quake3.
I'm using the 2.6.0-test5 kernel and it is much more stable regarding this particular problem than the earlier -testx 2.6 kernels were. I haven't had this happen on my 2.4.20 or 2.4.21 kernels however, so I'm not convinced it is directly related to the drivers in any way.
When the system hangs like this (black screen) the kernel is still responding well enough to shutdown on ctrl-alt-del or external login. I think it is probably agp driver related but nothing solid on that. As soon as the system gets all 3 Xservers shut down the vga text mode operates correctly during the rest of my shutdown but telinit doesn't correct this problem so something additional is being shut down by the time it reverts to the virtual terminal.
GeF ti4400 at 4x nvAGP, driver 4496