It appears to be a missing feature. :-(
There is a bug filed for this issue: #505323, GDM's settings bar inaccessible with keyboard
See How do I subscribe to a bug for instruction on how to follow the progress of this bug. If you are already logged in to launchpad and ready to go, click "This bug affects me".
If you really need it, you can use a different display manager, kdm for the moment. Note that it has a big bundle of dependencies. You can also go ahead and ask a question here on how to set a keyboard shortcut that doesn't depend on gnome running.
There is a keyboard shortcut to shut down your system that is available all of the time, but it's rather tedious and complicated:
Alt+SysRq+(R, E, I, S, U, O).
That is, hold down alt and sysrq, then type
REISUB to reboot. The SysRq key is the Print Screen key on most keyboards (on some laptops, it's Fn +Print, or Alt+Fn+Print)
It's not a completely clean shutdown either (although it's good enough). I'm sorry to say I think that's as good as it gets at the moment.