I'm the one who started this thread and I see that at least one other person has had the same problem, so let me summarize my most recent experience with this isssue.
I have traced the origin of the problem for me to cookies. Specifically, cookies from other BB's. I visit three rpg forums: Mongoose, EN World, and RPG.net. I finally noticed that the problem with bolding of posts only occurred when I visited the Mongoose forum before EN World. Visiting RPG.net first seems to have no effect, but I believe that they use the same forum software as EN World. Mongoose uses phpBB to run their forum so my guess is that somehow their cookies are interfering with the cookies set by EN World (a sort of cookie monster issue). I now visit this site first and everything works fine. I'll have to try visiting Mongoose first and then deleting all cookies just to verify this. It would be nice for someone more knowledgeable about browsers and cookies to look into this and confirm/discredit this theory.
I hope this helps somewhat.
Elwood