DomainKeys & Digital IDs

Yahoo proposes DomainKeys - a new technology to prevent spam. This sounds interesting, but why inventing such a technology when a technology that could get us rid of spam for good has been around for years: regular digital signatures!? The problem is, of course, that a person that is not tech savvy would have a lot of trouble to go through the procedure of getting one at thawte.com, dowloading it in the browser and exporting it to the mail client. But what if major webmail providers were to provide free digital signatures for users (how costly can that be to them?), signatures that would integrate seemlessly into the webmail experience. It wouldn't take long for a vast majority of email users to have a digital ID (even without knowing it), and a day would soon come when all you need to do would be to block all messages with no digital signatures, put those with signatures you don't recognize in a temp folder, and have the rest served in your inbox. It's such a simple concept! I can find no other explanation to why it is not already in place but a conspiracy theory - I suspect that those that profit from spam in one way or another are pulling the strings.