Everything should be marked as read automatically that was posted before and possibly during your current session even if you didn't open the thread. That's how all cookie based forums work when they're working properly. There has been problems with UBB in the past where this was not the case, but that was usually due to a glitch more than normal functionality and was likely due to cnet's shared cookie login across different sites messing things up. Good thing is we don't have to worry about that here.
