A Cre8asite Forums thread asks the question, does search engines rank (or index) an individual forum post or a complete forum thread in the search results?
The simple answer is that search engines simply index pages. So a thread is made up of several posts on a single or several pages. For example, the Cre8asite Forums thread, can list 20 posts per page and then you have to click to the next page to view the 21st post from that thread. Each page, would rank differently, unless the search engine considers them similar enough.
There are some forums that enable a user to view a single post on a page. So if those pages are accessible to search engines, it is possible for a single post to rank and be indexed. However, most forum owners would likely prefer the whole thread be indexed and exclude the individual post, due to duplicate content issues.
Bill Slawski explains in his post in the thread that there is a patent application on Agent Rank where the search engine will break up components of the page and rank each component on its own merit.
Imagine a system that instead of ranking content on a page level, breaks those pages down and looks at smaller content items on those pages, which it associates with digital signatures. Content creators could be given reputation scores, which could influence the rankings of pages where their content appears, or which they own, edit, or endorse.
Now, this is probably not used much today - if at all. But Bill says, "they may, or may not be a good match for a forum - but it's possible that both could be - forum posts are usually written by different authors, and are separated from other posts."
Forum discussion at Cre8asite Forums.