The Google offering reminds me of Verisign.
Obviously it will cost money to get the certificate, which means most of the world will never sign up.
And all the time you might get emails from people who haven't signed up, you still have to implement the same antispam measures.
It may well help with legitimate bulk mail. I have a neighbour who does this for a living. He has a team of people who spend all day working out ways to get around customers' spam filters. This way, if a customer wants to receive bulk mailings from say Levi Jeans he can stick their certificate into his whitelist. But this is easily achieved right now, by giving Levi an unusual email address and putting that in your whitelist.