David Berlind's write-up of Judith Donath's presentation yesterday at the ID Mashup on signaling is well worth reading. Signalling is important for reputation. We don't have the infrastructure, at present, to easily pick up on signals and use them. Should I trust some one with an "edu" TLD in their email address more than a Hotmail account? Probably. Universities, as a rule, vet the people they give email addresses to. Hotmail, obviously, doesn't.
Part of the problem is that the signals that are there aren't easy to see. For example, why doesn't my email client (Mail.app) show the URL that a link inside an email message contains?