Wow! After a whole bunch of trial and error, I finally got a good fix for this bug! I've just pushed rdesktop-cli version 1.6.3-maemo1 to maemo extras, so it should show up there in about an hour. This new version correctly handles the power button being pressed, and the screen going black. If the power button is pressed, your session will come out of fullscreen, and be put in the taskbar, so you can see the power menu. If the screen goes black, the session will also come out of fullscreen.

Now, back to work on the frontend... :-P