Welcome to the club. Make noise about this bug so Nvidia allocates developers to stability, not just performance.
I've been experiencing the same bug since, well, pre-2880. APM enabled, but you aren't suspending (and know that suspending is a cause of system death). Random hard, hard freezes. AGP is disabled (or not present, as is the case with one poster). Occurs when doing nothing specific in X, (I only run gnome) -- scrolling, moving the mouse, opening a menu, etc -- nothing specific at all.
Do you get somewhat "repeatable" state crashes? E.G. - Crash when setting a specific preference in Mozilla, then reboot, fsck, etc, load Mozilla, go through the same steps to get to the Mozilla pref, set it again the same way, crash? This isn't a moz-specific problem, happens all the time with random apps. However, change the order of what you do and the crash might dissapear, and then you can't get it to happen again so you can post something to the nvidia techs.
My solution: pass "apm=off" at boot time to the kernel. It isn't pretty, and I use a laptop, so it sucks, but for desktops the only penalty is increased power consumption, as the kernel won't issue HLT instructions to idle processors to slow them down or conserve power.