Actually, Brian's suggestion to reinstall was a good one, and for a reason
I wouldn't have expected. I noticed when I started to reinstall that I was
using the i386 version instead of the amd64 (consistent with my chip). I'm
not sure what direct effect that would have on gnome-shell, but
reinstalling with the proper version corrected the problem. It also fixed
another more cosmetic problem with the gnome display (an ugly black blank
space initially at the bottom of the screen). I'm going to reboot a few
times, and if the problem doesn't recur update the bug report.
