I've had the privilege to work with Fred Chong and Gianpaolo Carraro, a pair of extremely bright guys who work in the Architecture Strategy group. They have a passion for SaaS and are applying themselves energetically to mapping out the space of SaaS application architectures. They realized that there is very little guidance or documentation on best practices for SaaS architectures in the community at large - while there is some shared knowledge and a set of conversations, no hard documentation exists in any comprehensive way.
Fred is now on a mission to build definitive guidance on how to grapple with tough issues like multi-tenancy (including defining what that means), security, customization, metadata, and operations readiness.
Fred has posted the outline of the SaaS architecture guidance book that he is developing on his blog, and I'm pasting some of the chapter outlines here as well. Fred is an expert in Identity Management and Web Services management (he literally wrote the book for WSM at Microsoft), so he knows what he is talking about.
2. Business Model
3. Application Architecture Overview
4. Scaling 101
5. Data Management
6. Tenant Management
7. Tenant Customization
8. Application and Data Security
9. Programmable Software Services
10. Programmable Software Service Consumption
11. Instrumentation and Monitoring
12. Configuration Management
14. Infrastructure Security
15. Operation Structure
If you're interested in this, drop by Fred's site and take a look - better yet, send him an email and tell him what you think.