How to Add Login window button on Toolbar?

I wan to leave my Powerbook secure when I leave it and I want to add a shortcut button to the toolbar where the Bluetooth, Airport, volume, etc are. Is there anyway of adding a button to it, rather than having to click the user name in the top right then click "Login window".

PS. Just to clarify if you go to Login Window do the users applications varry on running including downloads once on the login screen or are they paused or something.

Thanks

just set a hot corner for your screen saver an require password to resume