I wish I had a little LED at my desk that would sync with my IM status to show people when I was busy or available to chat online or in person. One of the earliest usability features in the online world but still not replicated in the real world.
Better, I find, is just talking at someone with a cultural expectation of high latency in their response, which is true of a lot of IRC channels. In many previous HN threads people have mentioned the chat room format being crucial for their work discussion; you can direct a question at someone, while allowing it to be publicly visible too so that someone else can step in.