Comment 29 for bug 474495

I have a similar (identical?) problem with intermittent panel freezes at boot. I have found ten other bugs with nearly identical trouble.
I have also discovered that I can recover my panel functionality with two steps that you may wish to try. This works 100% of the time on my machine.

Access a terminal (Ctl-Alt-F1) and execute the following command;

 $ killall gnome-panel

The second step is to return to the desktop (Ctl-Alt-F7) and open a "Run application" window (Alt-F2) and execute the command;

 $ gnome-panel --replace

My panels and system are completely functional now (except for missing weather apps). Do not use a desktop terminal for the second command - the process will hang in a terminal. See Bug #466118 for more comments.

Hope this works for you too...