On ubuntu 10.04, I choose System > Disconnect > Swich User, then at the login screen I can click Ctrl+Alt+F4 to get a command line over a black screen, & then click Alt+F7 to redirect back to my password protected desktop WITHOUT entering password!
How is it possible and how can I fix it?
You need to use "lock screen" instead. Switching the user doesn't log you out or lock the session, it just gives you a prompt to log in as someone else.
I just tried this in 11.04 and it still does it, and this is an intended feature I assume (abandoning or locking access to your terminal doesn't mean it will keep you logged out!), but it does have implications. I'd suggest taking concerns to the bug tracker if you're worried about any of said implications.
