Ask HN: What do you use for persistent chat?

We are trying to improve intergroup communication at my job. One of the ideas we have come up with is to create a persistant chat room (or channel) for the groups to chat on. Then when an issue comes up the groups all have a history of the conversation. Currently we use GTalk for chat and most of us use Adium for a chat client. The only requirement is that the chat room log has to be visible to all parties with at least 48 hours of history.

So the question is, what do you guys use for persistent chatrooms?