...can you perhaps decouple the main website from the forums somewhat?
Reason being, viewing the main page also logs you into the forums. If you don't want to view the forums, it will still clear the new posts list when you leave the site. I might pop in during the day to check if my story has been updated, and then find in the afternoon that the forums have been reset and I have to try and work out which threads have been updated.
Perhaps a flag that gets set when you visit the forum section, which lets it know to clear the new post list when you log out/timeout?