Igor Bilogrevic, Julien Freudiger, Emiliano De Cristofaro, and Ersin Uzun have an interesting article that Cryptome has made available on their site. The researchers tested a system for users to provide encrypted data that could be aggregated, thereby protecting some user privacy while still generating revenue for both data brokers and the users. They report that their system provided accurate aggregates with as few as 100 users, that it generated revenues for both data brokers and users, and that the overhead is low.
You can read about there study here (pdf).