Yippie! I get to tell you "I told you so."
About this time last year I suggested using digital identity on mail servers to cut down on spam. Yahoo! has added an excellent dimension with DomainKeys. They suggest ISPs publish their public keys via DNS since there are already control measures in place to ensure that only the domain owner can modify their DNS records. Thanks to Jeremy Zawodny via Simon Willison.
It's not fool proof, but it immediately gives Yahoo! a leg up on their competition. It will get harder for spammers to spoof Yahoo! addresses in the From: line. And competition may compel others among the big ISPs to adopt DomainKeys. Basically, Yahoo! is a big enough player that their endorsement may encourage wider adoption of the technique. I don't expect to see a drop in spam overnight, but it looks like a step in the right direction.