Did you read either of those links? They explain it pretty clearly along with some examples. This is a major no-no from a company that already has people worried about privacy, imo.
I disabled everything PA and imref posted. Google is welcome to come up with all the cockamamie ideas they want to, but doing what they've done here with buzz violates a pretty basic trust that I'm betting will have a lot of privacy-conscious gmail users looking for alternatives. I'm personally not going to be one of them... yet. But you can bet I'll be keeping an eye on this.