I have been testing the Simple:PressForum for the last few days on a test site and I have to say I am pretty impressed.
Unlike other forum solutions this seems to be fully compatible with WordPress ,heres an extract form their site
Simple:Press is a feature rich forum plugin for WordPress. It fully integrates into your WP based site utilising the same user records and database and displaying on a single WP page. It is fully customisable and comes with a number of skins and sets of icons to get your started. Current users have shown it to be fully scaleable whether your site membership numbers the tens or the thousands.
I just need to change the styling so it fits neatly into MyWordpress and then carry on investigating all the options to fine tune it.
Have a look and see what you think