Dick Hardt is giving a new talk at Defrag. He's talking about trust; his thesis is that trust defrags identity. Much of what's he's saying is right in line with the reputation work (PDF) my students and I have been working on. He makes a critical link to identity: identifiers bind personas together to increase trust.
Intuition doesn't work well online because of the absence of clues and the ability to create false context. Institutions haven't done much better. He brings up another key concept this is largely about accountability.
Key point: binding behavior from multiple sites together leads to better accountability. This doesn't mean that we can't have multiple personas just that we ought to be able to link them if we feel the reward is worth the loss of privacy.