This has come up before, but there is a change in the situation that we should possibly consider: OpenID, an open standard for decentralized single signin. It more or less works for both your points. Your ID on most sites would be a URL on your favorite site (IE http://perlmonks.org/?node=theorbtwo). There's no single point of failure -- each site going down means that users who use that ident server can't log in anywhere, but has no effect on those that don't need to use it.