Usually this is completely dependent on the system you are using for a
signin mechanism. If you're using Windows Authentication, then pretty much
IE will stay authenticated until it's closed (and sometimes until the
computer is shut down if it's being really stubborn).
If you're using the User Registration component to protect something such as
a forum, then you don't really need a logout link because, technically, the
user really isn't logged in. The user registration form primarily just
checks the user's username and password then moves them on. It doesn't
really do much else so the logout wouldn't be effective even if there was
one in this case.
Hope this helps,
Mark Fitzpatrick
Microsoft MVP - FrontPage