Comment 274 for bug 439448

Another workaround is to change the current theme (via system/preferences/appearance) to another one. Once done, the panel get drawn correctly and you can switch back to the old one. This approach has the advantage that it can be used also where the panel is locked down (as in UNE edition).

As a side note, if you have to switch off in a hurry and are not able to use any of the workarounds proposed here, I think it is safe to hit the power button: it should be linked via acpi messages to the shutdown dialog.