I have been tempted to start development of a XMPP app. Most likely using BOSH to do it all. I was even thinking of adding XMPP transport support so you can connect to AIM/MSN/Yahoo/ETC through XMPP, and maybe direct support for Facebook/GTalks since those use XMPP.
But its a big project and would take a lot of time to get a good quality version coded up solo.
But its a big project and would take a lot of time to get a good quality version coded up solo.