Once I've upgraded to Lion I had a new icon on my Menu Bar next to Spotlight. It was an outline of a mugshot similar like one of the Guest account.
When I clicked on that icon I was able to select "Login Window..." which basically locked my Mac.
For some reason this icon is gone. I don't use it frequently so I can't clearly say when it went missing but I'd say within last 10 days probably.
How do I get it back? I have no idea where to re-enable it.