It seems to be messed up somehow. When I visited today, it showed only "new" on only the forums that had messages that I hadn't read.
I made a couple of posts, then every thread was marked as "new". Logged out and back in and now NO threads are shown as "new".
Obviously the status should never change unless you actually read the post.
My first guess is that I've seen this problem when the software tries to mark the read/unread status with cookies rather than in the database. I don't know the particulars of this software in that regard but maybe have someone check to see if there's an option to store the read/unread status in the database rather than via cookies.