Comment 6 for bug 153995

This is happening to me, too, starting with my recent upgrade to Gutsy (final) from Feisty. Once I'm in a full-screen RDP session, I can't get back to Ubuntu in any sort of normal manner (CTRL-ALT-ENTER).

"Lossy" workarounds:
 *Log out of remote machine (ends the remote session, retains local session)
 *CTRL-ALT-Backspace (ends local session, retains remote session--once logged back in via RDP)

"Lossless" workaround.
 *This one's weird, but if you hold down CTRL-ALT-ENTER, and keep it down, you'll enter a Twilight Zone where it's rapidly flashing back and forth between Ubuntu and the remote session. While it's in this altered state, hit the Left arrow key and then let go of all four buttons. It's like roulette, so it might take several attempts, but eventually, you'll get to the desktop to the left of the one that's got the RDP session in it.

Using this technique, you would want to start out by opening the RDP session on its own desktop, so it's not obscuring any apps/windows beneath it.