Ah. Okay... I've tracked down the problem to synergy.
I use synergy quite a bit to share a keyboard and mouse between
multiple machines. So, it looks like this problem shows up if I start
the synergy client on my laptop. After that, even if I close the client,
the problem remains until I reboot.
Thanks for your comments. This does not appear to be a bug report as
such. We appreciate the difficulties you are facing, but it would make
more sense to raise your question in the support tracker.
http://launchpad.net/support
** Changed in: ubuntu
Sourcepackagename: gnome-desktop = None
**
