I'll be honest, I'm not overly keen on the idea... or not for this forum at least. I've had shoutbox's/chatbox's on some of my other forums which have worked, but some that have been a bit of a disaster!
In my opinion, it takes away from the purpose of the forum. If you want to reply to a post, you can do... if you want to chat to someone, there's a PM function. Don't think a chatroom is going to make any difference... and think that the forum would only suffer because of it!