Comment 6 for bug 24305

This happens since hoary, and I found it's related to X screensaver while rdesktop is running in foreground and not windows screensaver.

When you 'loose' the mouse and keyboard, do the following if you want to avoid restarting X:

Alt+F4 (to close rdesktop window)
Alt+F2 and run gnome-terminal
find and kill gnome-screensaver (in dapper) or xscreensaver in hoary/breezy.

This should give you control over your desktop again.