Chatboxes are death for forums, in my opinion. What you end up with is people nattering away to each other using the chatbox and completely ignoring the forums so that activity falls and the place looks dead to anyone visiting or thinking of signing up as a member.
We have social groups, visitor messages, private messages and the general discussion area for people to talk - a chatbox is not only unnecessary strain on the servers, it is also damn ugly and usually full of people saying lol all the time.
Steve could have an entirely different opinion, of course, but I doubt he'll be implementing a chatbox anytime soon.