BuddyPress.org

@21cdb (“The only thing is missing is the ability to choose if you want to post an “Update” or an “Discussion” (or Forum Post, or whatever you want to name it).”)

You could achieve that using categories and some if/else PHP. I did something like that in my P2 group blogs, that I use instead of forums. WordPress 3.0 will apparently have custom post types that will make this easier.