Where exactly do you see the problem? A request is made to a site where the user is authenticated already. The authentication info is passed along with the request (probably either a cookie or an HTTP authentication header). Usually in these circumstances the server will remember when the last access happened, and require re-login if the time passed since then is too long (say, half an hour). Whether the browser was used to access some other site in between doesn't really matter.
On this website the above situation does not arrise as ( i guess )
Do you mean JavaRanch? It works just the same here - have a look at the cookies stored on your system for saloon.javaranch.com. You can even close the browser, and still don't need to re-login the next day.
[ April 11, 2006: Message edited by: Ulf Dittmer ]