I heard about AOL and Yahoo's plans to charge for email delivery today on the Digital Experience Podcast. It appears that AOL and Yahoo will charge email senders if they want to guarantee that the message will get to the recipient. If the sender doesn't pay, the email may be stopped by the spam filter, have the attachments stripped, or have the urls removed.
I wonder how successful this will be. If I were an AOL or Yahoo customer, I would be upset if mail that is destined for me is not delivered. Also as a sender, I am not going to pay to send email. This is the same way I feel about challenge-response spam filters; I will not respond to a email challenge.