Hi Chris,
the session has nothing to do with login or logout (although you can _force_ a session change on login if you like.)
The session is bound to the instance of the browser. So you need to close the browser (*) to get a new session number.
(*) note, in IE this means closing just the current instance of the browser, in FF it means closing _all_ instances of the browser that are open, and so on - it differs from one browser to another.
cheers
Bruce