Alfred,
Interestingly we were discussing this kind of automatic linking today on the IRC channel. I am finishing up a "trackback" http://www.movabletype.org/docs/mttrackback.html package that will allow any object to be notified when a link is generated to that object.
Documentation pages are indexed as static pages, and forums postings are both objects.
So this is a partial solution. An interesting feature is that this will allow linking to and from off-site content as well.