Now, users can register for Facebook using their Gmail accounts. This is a quicker, more streamlined way for new users to register for the site, find their friends, and start exploring.
Existing and new users can now link their Facebook accounts with their Gmail accounts or with accounts from [other] OpenID providers that support automatic login. Once a user links his or her account with a Gmail address or an OpenID URL, logs in to that account, then goes to Facebook, that user will already be logged in to Facebook.
Doesn’t cost them anything, but I’m still surprised they’re doing it. Interesting that the Google confirmation prompt mentions they want access to your Google Contacts.