It behaves all different kinds of way - but it seems to me that whichever forum you go to first shows correctly and the other shows everything is new.
I've finally adopted a method of going to FT Miles first, since that's where I read the most forums and therefore most want to differentiate new posts. Then when I flip over to FT Travel, I don't care so much that all the folders are yellow, since I only check 3 forums there. What continues to irritate me however, is that if I click on an external hyperlink within a thread on FT Miles, and then go back to the top page on FT Miles, now all the folders are white, indicating no new posts at all, and I can't tell what's new.
FWIW, it's been briefly discussed before:
http://www.flyertalk.com/forum/Forum72/HTML/000562.html
[This message has been edited by svpii (edited 07-10-2002).]