The MSDN Solution Architecture Center recently went live with a new section that focuses entirely on the development architecture for SaaS applications and there are already some great articles on there.  I would highly recommend having a look at "Architecture Strategies for Catching the Long Tail", it's a great introduction to what exactly Software as a Service is and speaks to many of the complexities that differentiate this develoment approach and delivery model from the more traditional methods that we are familiar with. This will prove to be an essential resource for all those interested in SaaS, and particularly useful for developers. 

http://msdn.microsoft.com/architecture/saas/

--Rob.