why would you log out?

In Windows you'd hit Windows-L, which would take you to the login screen, and then they'd click "Guest" (or whatever alternate login you've set up). You'd still be logged in, and when they were done (or were giving it back to you for five minutes) you press Windows-L again and choose your own login to switch back to your still-running programs.

I'd be astounding if Linux didn't have an equivalent.

Ubuntu already has a "proper" guest mode, only accessible when unlocked by a regular user, wiped clear each time and only providing access to the guest sandbox. It's available from the system menu, so two clicks to activate when logged in already, or a couple of clicks and a password if the machine is locked (and in that case you don't have to go through your regular desktop to get to it).

How about using Opera kiosk mode - it appears to meet your requested features.

Also FastUserSwitching doesnt require logout IIRC.

os x has a guest mode. (enable it as documented here http://docs.info.apple.com/article.html?path=Mac/10.6/en/156...) just tell them to wiggle the mouse, click "switch user", and click "guest".