what is the shortcut to force a logout?

When I have this problem, I simply Ctrl + Alt + F2 into a terminal, log in, and type

killall -u  [username] gnome-session

This will basically kill all of your processes and log you out. If you want to logout all users at the same time in this manner, then you leave out the -u [username] part.

I guess you could create a new keyboard shortcut that runs this command. The only problem with this that I can forsee is if the freeze-up is because of gnome itself. Then it may not recognize the shortcut.

It seems that the latest update added an "options" menu on the keyboard preferences-->layouts that lets me activate the "control-alt-delete" shortcut like before. Or it might have always been there and i missed it.

You can open Keyboard shortcuts and create a new shortcut. Run the command killall -u yourusername gnome-session.