I'd like to input my two cents: The reason that twitter and other sites have the hashtag is to provide metadata for the content because there is no inherent organization on those sites. Twitter is like a giant wall of text that you just keep adding lines to, with hashtags allowing you to group similar lines. Forums have a built-in way to organize things: subforums, sub-subforums, etc. Ideally all relevant content will be posted in appropriate forums.
I'm not saying it's a bad idea, I'm just a little wary of it's inherent usefulness in an already-organized platform.