Comment 1 for bug 487870

I believe you need to add the Fast-user-switcher to the notification area. I'm afraid I can't tell you how to add things, it's supposed to be as simple as right clicking on a blank area of the notification area. In the UNR it's nigh on impossible to find the single pixel where this behaviour seems to take effect.

It was originally enabled by default in the alpha of Karmic, it seemed to disappear later on in the release cycle as I upgraded.

Jonathon