I was doing some research for our new book, The Salesforce Handbook, and came across a whitepaper entitled,
The Force.com Multitenant Architecture
Understanding the Design of Salesforce.coms Internet Application Development Platform
This whitepaper is a great read if you are interested in the guts of the Force.com platform and how it works. It talks about multitenant application design and metadata driven architectures from a high level. It offers some really great insights on how data is stored and retrieved on the platform and well as application development, logic and processing.
You can download the whitepaper here.