I believe you just "X" out of the window. I asked this question on another forum recently and was told there was no "official" way to log out, though I don't know if that's correct or not. I agree though, seems like a security risk.
But because when you first login it saves a cookie in browser, even if you close that window and try loging in again, it does not ask you for login info...
This is a big security risk, as someone using your computer can easily mess up the server.
If you hit the 'X' you don't always get logged out.
I sometimes use a public library computer every tuesday in my lunch break.. and everytime I log off these forums or any other forums, and my cpanel/whm for that matter, I'm not actually being logged out.
WHM is problem, because it doesn't log you out striaght away. I've had this problem.
I've actually tailored the logout screen of my cpanel skin to tell my clients that if they're using a public terminal or other unsecured computer, that they should completely shut down the browser after they've finished their control panel session. I made that modification after seeing notes in a few different forums about this very issue.