[Dave] first brought Multi-Tenant Data Architecture MSDN Article to my attention a few months ago. Unfortunately, I've had this article in my items to review folder. Anyway, I finally read the article in detail and thought it quite valuable. Basically, the article describes several approaches for designing a Multi-Tenant Data Architecture. It reviews each approach and reviews the pros and cons.
[RoyAshbrook] blogged his thoughts on Multi-Tenant Data Architecture.