I've created a theme for WordPress that turns a WordPress blog into a forum. You can see it in action here.
It contains these features:
- Lists Posts by date of last comment (instead of published date) (included as a page template)
- Unread topics are highlighted and can be used to filter posts list (included as a page template)
- A hierarchical Forum list (included as a page template)
- AJAX Comment Preview (can be turned off)
- Comment QuickTags (can be turned off)
- Forum-looking Layout
- Pages and Categories can be used as Tabs (configurable)
- RSS 2.0 Feed of forum posts (can optionally be included in the theme or replace the main post RSS feed link)
I'd be interested to know if there is any interest for me to release the theme.
It is currently dependant on at least one plugin, but I plan to add another page template that removes the dependancy. You can read some more about it @ my blog.