Same Thing, Other Side
I'm experiencing the same issue in my latest installation of FreeBSD 8.1 except that I am not using an nvidia card (or driver) on my system. The video card is an onboard (ATI Radeon 3000) and the problem has occured in both i386 and amd64 versions of FreeBSD 8.1 now. It did happen once, during xscreensaver although I agree it is not limited to the screensaver itself. I'm using fluxbox on xorg so no KDE components to speak of here.
I'm inclined to say that this issue may be due to ACPI settings or maybe even a GENERIC kernel attempting to use today's hardware. IDE vs SATA for example. I've seen issues like this in the past where the OS attempts to communicate with the hard drive using incorrect read/write modes and causes the system to lock up which may account for poor log files being written during a crash.
Anyways, just my two-bits. Hope it helps.