Update #1: I was sloppy above. It's not Mint that is rebooting; it's the X server. This is apparently a common problem,although the variety of symptoms posted there suggest that this is in fact multiple distinct bugs with a common end result (X crashing). Things I have discovered:
- Unlike several of those reports, I have not experienced any crashes as a result of clicking. Only typing triggers a crash.
- Unlike the reports from laptop users, power management is not the culprit (the machine that crashes for me is a desktop).
- Someone pointed the finger at running the AMD-64 version of Natty on an Intel CPU. My copy of Mint is in fact based on the AMD-64 Natty, but my box has an AMD-64 CPU.
- Turning the proprietary nVidia driver off and then on did not help. (I have a GPU from nVidia.)
- Installing xserver-xorg-video-nv did not help.
Update #3: See this post for earlier problems and this post for a later round of nVidia issues (and how I resolved them).