This might have some promise … provided the participants don’t get harvested to harshly:
SpamArchive.org is a community resource that provides a database of known spam to be used for testing, developing, and benchmarking anti-spam tools. The goal of this project is to provide a large repository of spam that can be used by researchers and tool developers. In the past, there were a few small personal spam archives that were used. There was no large set of spam that could be used to test new anti-spam algorithms. Thus, developers could not sufficiently test their techniques across a range of messages. Also, the lack of a “standard” sample of spam made it difficult to effectively benchmark anti-spam tools.