Great article by Richard Tooker in October's Direct titled Prove It: An enterprise-wide control group can help measure a database's value.
Almost all marketers employ control cells to measure test lift. Tooker advocates establishing an additional holdout control panel of 50k customers who receive no marketing communication -- ever. This panel allows a company to determine the aggregate lift of their customer communication efforts. The panel answers the question, "What we getting from all of our housefile marketing investment?"
Worth reading and considering.