BoxBe is a startup you should look at...they started with this idea, but according to a few friends of mine that use it no one has ever bothered to pay them for their attention. As a revenue generating business model, it seems thats spammers don't pay!

btw - Goodmail has signed up ISP customers to get mailers to pay a couple of cents per message to "guarantee" the delivery of the message. To me seems like a protection racket of corp. customers by the ISP and Goodmail.