Since we are on the subject, I don't know if you saw it the other day but there was an excellent New York Times article by Juliet Lapidos titled What the NSA knows about you. The Immersion program from M.I.T. I just signed up, will report back later.
“[Immersion] asks users for their Gmail address and password; it then scans every e-mail in their accounts and scrapes the metadata to create a portrait of their personal network. With the circles and lines of a network diagram, it highlights the 100 people with whom you’ve communicated most, and shows how closely they’re connected to you and how thickly interconnected with one another in your mailbox.”