I think a chatroom is a great idea.
I am concerned that instant messaging might detract from thread content. Someone might just ask a friend rather than posting something which could later be used as a reference for the community. I don't think a chatroom (or perhaps even a system of chatrooms--plant problems, smoker's lounge, political corner) would detract from content in that way, and actually could present a path to some new and interesting community elements (and threads).
There is one glaring issue with this, and that is moderation. Modding a chatroom is a totally different beast than modding threads--and there is a much greater opportunity for hackers, dickheads, weirdos, and even law enforcement there.
As an at your own risk kind of thing, though. I think it could be really beneficial--great idea imo dude!