I personally, have never seen a chatroom work on a forum (I moderate several forums and I actually run the IRC Chat server for one of them, no one uses it). Like beardie said, unless you coordinate an event, people usually come in, see the place is empty, and leave.
What I've seen work better than a chatroom, is simply creating a thread in the Off Topic section that is for random chit chat. It tends to have a better flow because people can come in and out at their leisure and can easily catch up on what's been talked about. Even if no one is currently in the thread, there is still some history you can go back and read, as opposed to walking into a totally empty chatroom.