Goodmail Systems has been getting lots of press lately. They're offering CertifiedEmail, which is simultaneously a reputation, authentication, and payment system. They investigate potential customers, and refuse to serve spammers. They sign a customer's public key with their public key, and allow the customer to send signed emails. The mailbox provider verifies the signature and gets paid a little bit for their trouble.
Seems that people are wondering how this will reduce spam. It looks like a solution to false positives in mailbox providers' filters, not a solution to spam.
Ah! But what if you kept the false positive rate the same? That would mean that you could tighten down your filters in proportion to the good mail (systems) that bypasses your filters. Lower your bayesian or Spam Assassin thresholds because you know that a greater percentage of that email stream will be spam.
You could do the same thing using DomainKeys. That's where the most likely competitor to Goodmail will come from. Commerce usually increases when there is competition for two reasons: first, because customers think "Gee, there must be something to that system if two separate entities are trying to solve that problem the same way." Second, customers are more likely to spend money in a competitive free market, because while they aren't likely to be able to evaluate the proper fee to charge for signed email, a competitor will be.
People don't need to be valuation experts to buy something in a competitive market; the competitors do the valuation.