WOW! I was going to make this EXACT same posting earlier today, but never got around to it.
Yesterday I contacted a friend that is a forum administrator at a different forum that uses the same vbulletin software to see if he knew why this was happening over here, but he could not find any setting for it in his system.
I am wondering if there is not some sort of setting that is "marking all threads as Read" upon each visit, but unfortunately, with either a short timeout or by posting a new reply, the system must be assuming that you have started a new session. I don't know.
There are at least 4 forums that I visit that use the vBulletin software, but this is the only forum that behaves this way. Not only does it mark the thread as read, but then it removes the little button allowing you to jump to the first unread posting.
I hope we can get this fixed.
Just a quick followup, I notice this most when I make a posting to a thread, and when I click on the "Quick Links", "Today's Posts" to finish reading the rest of the forum, then all of the threads show up as being read.