The forums have been updated to handle new unread posts thusly:
Clicking on the envelope icon in the leftmost column will take you to either the first unread post (in the case of new unread posts in a thread) or the first post of the thread (in the case of no new unread posts)
You can discern if there are new unread posts in any thread by either an opened envelope that is "ready for reading" or a bold thread title
Thread titles will now only link to the first post in a thread
In short, the envelope has now become a more descriptive and larger TINY BLUE SQUARE.
These changes are now finalized. Sorry for the interim confusion.