Anil Dash has posted about my forums powered by Movable Type. I started doing this more than a year ago, and I have evolved it on some sites into active forums with additional features. For example, take a look at Seinfeld Forum. As you can see, this Movable Type-powered forum has over 4,000 topics (entries) and over 15,000 replies (comments). Features include a "post count" (using the CommentLeaders plugin) and an associated status level based on post count. I recently added automication pagination of threads using my MTDynamic plugin (although I don't think I have released that version of the plugin yet). I have also added a search function that searches only the forum section of the site (using a source-modified version of MT search). Another active forum example is Mars Forum -- over 20,000 replies (comments) on that one.
I have not yet posted this to MT hacks yet, mainly because it would take a long time to document all of the steps to get this set up. I do plan to do that, but I have been very busy. Let me know if you are interested (extra motivation may help me find the time :) )