Create an run-only script with the Script Editor with the following:
tell application "ScreenSaverEngine"
Set an Quicksilver trigger to run this script and map the shortcut key to what you like (I used ctrl-command-delete).
If you have set the screen saver to require a password, you now have an keyboard short cut to locking your screen.
You can quickly put your Mac to sleep with the following key press tip:ReplyDelete
Cmd + Alt + Eject