Brad Fitzpatrick Explains Goog's New "Social Graph" API, In Plain English

Before you get yourself in a privacy tizzy-- the API can only access public data:

"The API returns web addresses of public pages and publicly declared connections between them. The API cannot access non-public information, such as private profile pages or websites accessible to a limited group of friends."

Play with demo applications here and catch a glimpse of what can be done with the API.