Actually, when you click "Mark all posts read" on the main forums page, it only marks the posts that have been posted by the time you logged in. Therefore, the next time you log into the CoasterBuzz forums and go to a thread, click the "Scroll to newest post" link, which is above the thread itself. It'll jump right down to the last post you "read."
I don't know how well I explained it, but give it a try.
-------------
Matt Lynch
Co-Webmaster, Kennywood Boulevard
http://kennywood.coasterbuzz.com