Sure - there are lots. Phpbb is the one that seems to get used the most. There's discussion of phpbb and some other discussion board software in the archives of this forum.
A quick google search turns up this link.
To be honest, I don't recall the exact reasons. I remember that when we were looking at it they had just started supporting Debian, and so there were concerns about whether it would work on our system at all. Also, haven't looked into the price in a while, but I think that the cost was fairly high, with fairly low demand from our customers.
It looks like Sun has purchased Chilisoft, so it's possible that the pricing structure has changed. I will run it by our team, but I don't think we'll be implementing this anytime in the near future, though it would be a nice thing to support. My guess, though, is that it would probably be better to run ASP software on a Microsoft platform (or, better, not at all :>).