According to Dan Gillmor, AT&T has received a patent for a way to defeat anti-spam measures.

"A system and method for circumventing schemes that use duplication detection to detect and block unsolicited e-mail (spam.) An address on a list is assigned to one of m sublists, where m is an integer that is greater than one. A set of m different messages are created. A different message from the set of m different messages is sent to the addresses on each sublist. In this way, spam countermeasures based upon duplicate detection schemes are foiled"

Nice. Thank you AT&T. Though I wonder how effective that method would be. It doesn't seem like it would scale that well.