I had the same problem after upgrading to 11.04 from 10.10.
The fix is simple. Just go to: System->Preferences->Keyboard Shortcuts. Scroll down to the "Window Management" Section. If you find the action for "Move between windows, using a popup window" you will find that the shortcut shows as "disabled". If you click on the "disabled" and then hit Alt-Tab it will register that combination. Now it will immediately start working.
Hope this works for you.