Clicking Logout logs you out of Tomato's (Linux OS) firmware and clears your user session. This will take you back to the initial login screen, where you're prompted to enter your logon credentials again. This sometimes causes confusion, as people believe that they “need to log in in order to log out”. This of course is not true.
Once you see the username and password prompt (and a ?), you are logged out. At this point, if you click Cancel, you will end up at the (401) “Unauthorized” page. This option is not supported on MS Internet Explorer Version 7 and the “logoff” item does not show in the menu. In this case, you will need to close the browser completely in order to log out.