Comment 3 for bug 671423

I had the same problem on a fresh German Ubuntu 10.10 installation. As described above, toggling the 'show hide buttons' solved the problem for the active session. The problem appeared direct after boot up.
I found some hints to reinstall the indicator-applet-session package. I did so, in the synaptic package manager I marked it for reinstallation and applied it, and the problem didn't appeare now for two days.