another obvious one would be to give instant notification of mails via jabber, like google talk.
More interesting would be able to seamlesly able to im the contact instead of mailing. let's say i am about to mail you about some thing, and when I start to compose, rather than sending a full mail, I should be able to im. (for example the address drop down will have an icon indicating that you are online and I can click that to open an im session in the browser itself aka meebo.com)
Think of it like quick email exchanges, and the whole thing can be archived as a conversation.
IM by it's nature is email fastforwaded. When I want to talk to some one, if I can im them first, that should get priority.
btw, this is my first post in the forum. I am terribly excited about this. I have a project to migrate about 13,000 accouts from an iPlanet installation and was just looking around for an AJAX based imap client, but found some thing truly out of this world. Way to go people. My best wishes!
raj