The idea of payment systems for email has been around for a long time. The concept is simple: you pay someone for the electronic equivalent to a postage stamp, you include that with the email message (affixed and encoded in a way that prevents it from being faked or stolen), and somewhere on the receiving side, if they accept these particular stamps, your mail gets preferred treatment. The idea has been around for a long time, but has not caught on much. That part may now be changing.
You may have heard already that AOL and Yahoo!, who announced agreements with Goodmail Systems back in October, have now announced that they will start deploying Goodmail's payment system, and will make it an integral part of their services. You can see the New York Times coverage of this latest announcement. I have no specific knowledge of what's going on in this case, so here's a general analysis of this, given as someone who works on antispam stuff.
First, I think it's quite time to try this on a major scale. In our paper A Multifaceted Approach to Spam Reduction, my colleague Nathaniel Borenstein and I outline a broad array of antispam techniques, and we include payment systems in that array. I believe that payment systems alone are not a solution, but that such systems play a part in an overall solution. I'm glad to see two major ISPs trying this out, and I'm eager to see how well it works.
Second, I do not believe that this is the beginning of the end of free email. AOL, for example, appears only to be replacing its existing IP whitelist with this payment system. That is, it's saying that if you want to bypass their more aggressive filtering, you can buy a postage stamp. If you want to send free email to AOL users, you still can, but your mail will be filtered just as it is now. I think there is enough of a strong sentiment in favour of free email that it will stay.
Third, I am concerned about the effects that are likely as many companies join the world of payment-system providers. There would have to be some solid cross-acceptance of stamps, and I can foresee problems if this doesn't happen smoothly. At some level, this is analogous to acceptance of national postage stamps by other countries, so it is possible for it to work. It's also possible for it to create difficulties, confusion, and disillusionment with the systems.