> xmpp ties your identity to the server you register with, when there's technology to tie your identity to a cryptographic hash (you can use DHT to find people)

I have to disagree on this. XMPP follows the same model as email, where your identity is tied to a domain name, and I think it is about as good model as possible in our current internet. The key is that you really really should have your own domain, which then allows you to switch service providers at will without disrupting your identity. Of course I admit that it is bit idealistic viewpoint.