The Jabber chatrooms that we run at ietf.xmpp.org and irtf.xmpp.org are always popular during IETF meetings. They seem to have two main uses:
Semi-official side channel during WG sessions. For example, during the second MARID session (which I scribed), people were listening in to the audio feeds and used the Jabber room as a way to ask questions, which were relayed IRL by someone in the meeting room (often by WG co-chair Marshall Rose, who was following the chatroom discussion while running the real meeting).
Utterly unofficial undernet during plenary sessions. The meeting room of choice is called "badattitude" and lives up to its name. The discussion here is freewheeling and irreverent -- something like Mystery Science Theater 3000 but with plenary speakers instead of bad movies as the objects of ridicule. It's this room that had Tim Bray worried about some of the forthcoming process changes within the IETF, but it's best not to read too much into such yammering -- I saw it mostly as a way to let off steam.
The highlight was when IETF chair Harald Alvestrand went up to the podium to give his talk before 1200 or so people during the second plenary session on Thursday night, since he joked that he was doing what seems to be the first order of business for all the speakers: minimizing the Jabber chat room! (BTW, I spotted an Exodus icon in his system tray -- go pgm!)