A prediction: You know what will replace CAPTCHAs? A credit card number. That's right. Sending an email will cost 1/100th of a penny. A craiglist posting will cost 1/10th of a penny, etc, etc. Someone just needs to make it work.
The ideal, once your email-receiving price gets high enough, would be for spammers to basically send the kind of spam that you would have paid to know about anyway (e.g. instead of reading a gadget blog, spammer pays you $1 to look at his customized spiel pitching a particular gadget similar to the ones you've previously liked).