I'm having the same problem but I can usually reboot it remotely.
When it froze today, I left the computer for about 45 minutes and when I came back the screensaver was running and the keyboard and mouse were responding again. It had somehow recovered. I also had this message waiting:
[319892.477092] NVRM: Xid (0000:06:00): 32, Channel ID 0000000b intr 00040000