The guys from Microsoft Redmond put together a code project that illustrate how to build multi-tenancy Azure applications.
Below is an abstract of the project,
“The Cloud Ninja Project is a Windows Azure multi-tenant sample application demonstrating metering and automated scaling concepts, as well as some common multi-tenant features such as automated provisioning and federated identity. This sample was developed by the Azure Incubation Team in the Developer & Platform Evangelism group at Microsoft in collaboration with Full Scale 180. One of the primary goals throughout the project was to keep the code simple and easy to follow, making it easy for anyone looking through the application to follow the logic without having to spend a great deal of time trying to determine what’s being called or have to install and debug to understand the logic.”
Check it out here at http://cloudninja.codeplex.com/