On the new Software as a Service (SaaS) resource center over at the MSDN Solution Architecture center has a pretty interesting article on Multi-Tenant Data Architecture by Frederick Chong, Gianpaolo Carraro, and Roger Wolter.

The article covers all the basic requirements for SaaS applications that need to support multi-tenant data (for example, a hoster ERP solution) and the various options and tradeoffs in ensuring the security and scalability of the software service, like having a database for each tenant, or sharing one for all the data. The article also covers some basic extensibily options at the data model to deal with tenant-specific extension fields.