I believe Howson's contract was 'up' this summer, allowing either side to walk away. If neither did, it was renewed for another two years. So, to my understanding, he's under contract until June 30, 2013.
If I'm remember my college-level law class from many years back, that's what an evergreen would basically consist of. Either side can walk away during a certain period, and if neither does, it automatically renews.