Edit: A new, longer version of this post is available here.
I think we need to see some developments for forums and bulletin board software regarding the Semantic Web. Yahoo SearchMonkey and Google Rich Snippets are already indexing semantic content from the Web, and you can be sure that Bing will go the same way if recent developments are anything to go by.
Drupal is about to get the jump on vBulletin and phpBB by becoming one of the first forum (content) systems that will have semantic markup out of the box allowing search engines to know what is a post, a reply, a user / person, a topic, etc. And not just for the last 15 posts either a la RSS, but for all site content.
On October 19th, Dries Buytaert committed RDFa additions to Drupal 7 core into the code repository (Twitter announcement). RDFa is a microcontent format based on Semantic Web technologies, that allows people to embed semantic information in their web pages.
So I propose that when vBulletin 4 comes out, someone grabs the default set of templates, and marks them up with RDFa just as was done for Drupal.
Then, whoever wants to benefit from having semantic markup identifying the posts, replies, users, topics and forums on their multi-forum website can do so.
RDFa and search engines (phpBB SEO)