Read about this concept on Slashdot last week and just found a link to the a site that describes it in more detail:
“IM2000 is a project to design a new Internet mail infrastructure around the following concept: Mail storage is the sender’s responsibility.”
Looks similar to IMAP and quite efficient since mail can be filtered before it gets anywhere near you and puts more responsibility on the senders. It has some noticable flaws, but could be a step in the right direction to curb unsolicited mail.