Yahoo! plans to fight spam with 'domain keys'

So apparently they want to use a signature based system, which means that all mailservers would have to be updated to work with the new system. I guess they can push this (or else all of our x million yahoo accounts will reject your mail), but what happens if the ISPs say who cares about sending mail to Yahoo?

However, how is this going to work for the ones that use their ISPs mailservers, but use a different domain? Guess we are going to see... So every domain will have to have their own domain or their own key? Which central keyserver is going to hold those keys.

And what keeps a spammer from purchasing a real email account from someplace, spam from there - will the entire ISP's key be blocked then?

And of course the solution would really have to be open source...