Portable identities as a feature is definitely being worked on, but IIRC it will only help in the "I need to move from one instance to another" case. I don't have a project issue number handy, but it's there.
Multiple identities seems like a problem best solved by client-side tools (e.g. tweetdeck handles this usage really well IME).
Multiple identities seems like a problem best solved by client-side tools (e.g. tweetdeck handles this usage really well IME).