It tracks on a per session basis. So while your session is active once you visit a forum, the main page will show all topics in the forum as read, even though you might not have read them all. It will show any new ones since your last visit to that forum.
If your session expires (you're gone for a certain amount of time), then it will show all unread again.
We have a few ideas on how to improve this for version 8, since it always causes a bit of confusion.
