Bram: I don't think that using email addresses as identifier for instant messaging is a good idea. Email addresses change sometimes, leak information about your identity and depend on the evil DNS.
It is much better to use the hash of your public key as identifier. This has a lot of advantages: The protocol can be made secure very easily: somebody who wants to communicate with you can just look up your host key and check the hash. With email addresses he has to fetch the key and must somehow make sure that he has the correct key which isn't really easy. Another advantage is that you can have as many addresses (pseudonyms) as you like.