You should check out buddycloud.org. I'm not in any way connected to it, but I saw it mentioned on HN a couple months back. It is based on XMPP for the backend, but the documentation indicates a webclient, as well as mobile clients. Their documentation seems quite extensive and it was part of Google Summer of Code this past summer.

I'd be interested in hearing other people's take unless it involves "XMPP is too complicated."