I've heard that proposition before, and there are a few things about it which make me nervous.

First and foremost, mandatory whitelisting, as you propose, should be the last resort. It centralizes the mail system and severely limits my freedom to send mail to whom I want from where I want.

Even if such a scheme is implemented, it's possible all it would do is force spammers to become more efficient and slightly less profitable. More efficient in that they would only send mail to known recipients, no more of this dictionary stuff -- and that's assuming these trusted relays charge even for refused connections (which would be controversial). Less profitable if they eat the relay costs themselves instead of billing their clients for all or part of them.

The best thing would be if 1) ISPs do a better job at recognizing and blocking zombie PCs sending spam, and 2) if naive business would stop paying "email marketers" thinking they will make new customers (I can't believe enough people respond to spam that they actually do... but maybe I'm wrong, in which case number 3) would be naive people not responding to spam advertisements).