The way social apps can authenticate against each other has taken usability to a whole new level. But it makes you think how cataclysmic a security breach on one account could be. The new iPhone Blog-it app, for instance, easily created links with my Yahoo, Facebook, Twitter and Google identities.
It’s a risk/reward issue, though. The risks of this interoperability are relatively low (access to one only gives limited access to others – just posting in this case), while the rewards are great, in terms of maximum accessibility.
OAuth, which underlies quite a lot of this, is very good at restricting what one app can do in another.