In my opinion there can be one <h1> for the website headline (like here "my little forum") and another one for the article/document/posting headline - and even more if there are several articles with the same relevance on one page.
But you're right: an opened complete thread is a hierarchical structure so the initial posting headline should be marked up with <h1>, the headlines of the replies with <h2>.

IMHO, only one <h1> should be used - although this is controversial. A little reader:
http://www.w3.org/QA/Tips/Use_h1_for_Title
http://meyerweb.com/eric/thoughts/2004/07/21/pick-a-heading/
http://meiert.com/de/publications/articles/20070106/ (in German)