Authorization ( we can reuse openid ,oauth ..)
* This is not same as some server giving user data api's like google user data or microsoft's contacts api but its the other way round.
Browser is the best place to start with ...... Weave looked like taking baby steps towards it but its roadmap seems to be different and its motto too.
Demand is showing that here lies an opportunity for a social client-server app running on an open standard. Functioning much like an email client which acts as an interface to your mail via SMTP or IMAP, This will be the one centralized location for all your networking, messaging, blogging and personal publishing needs. http://rolfskyberg.wordpress.com/2007/06/29/the-time-for-a-social-client-is-near/