It just happened again, with the new Nvidia driver, 8.1-Release and KDE 4.5.2. Now my machine has never been under heavy use when this happens. Again, I cannot find any sort of crash log or log file mentioning the freeze. Not sure how we can try to diagnose the problem.
How long have you been crash-free on 9-current? I hate upgrade to that as other stability issues may arise and the problem is intermittent on my machine (uptime was 4 or 5 days this time). If this continues, I'll move to another window manager and see if it still happens. I will try to reboot with the nv module instead of Nvidia, but if I remember right there were screen issues with it and I may not be able to run it long enough to see the freeze (i.e., need to get work done).