With nowhere near the fanfare accompanying SenderID record checking in MSN and Hotmail, Yahoo! quietly turned on DomainKey checking the other day.
Yahoo! appears to be taking smaller steps than Microsoft, which I think is a good thing. The presence of a valid DomainKey record is displayed, but bad or missing records don't appear to be shown or penalized. This is a nice contrast to Microsoft's somewhat abrupt requirement that SenderID be included for all mail into its servers.
For marketers, these two events mean that authentication is here, and it's here now. The first and best impact of these technologies is to take a bite out of phishing by pointing out clearly fraudulent email. Longer term however, these systems will provide a necessary component for the next generation of reputation systems.