The Web is abuzz with discussion of AOL's instant messaging patent. I haven't read the patent's claims myself, but the News.com story on it says it covers "anything resembling a network that lets multiple IM users see when other people are present and then communicate with them."
That quote is a rather good description of MIT's Zephyr system, which had been around for years before I used it as a freshman at Stanford in 1996. I wonder if the patent application mentions Zephyr as prior art. If it doesn't, the patent is probably largely invalid. If it does, the claims probably cover far less than the press coverage implies.