You have got to love the ability of spammers to adopt new measures to get their mail read. Microsoft has been pushing SenderID, but now spammers are using SenderID headers to get through the filters.
When I attended a Privacy conference about four years ago in Quebec, a Microsoft rep was there talking about how sender authentication and public-key infrastructure was the answer. He was roundly panned by every single one of the other panelists as being out of touch with reality and with confusing Identification with Authorization – two totally different concepts. Authorization does not rely on a positive identification of the sender/human.
Until email costs money to send, we will continue to be barraged with spam.