No Trust to X

Ctrl+Alt+<Keypad *>  - this is all you need nowadays to bypass screen lockers in X Server. Although, I believe, this flaw will be fixed pretty soon, the trust is lost for ever, and I'm going back to roots by switching to X-independent "vlock".

"sudo vlock -na" works even when called from X, and seems to be the most secure way to lock your machine, since virtual consoles are handled as well. 
 (Make sure you remember root password before executing the above command.)