Here are 2 good articles on SaaS:
1. Is the SaaS Model a Friend or Foe? by Jeff Kaplan
2. Architecture Strategies for Catching the Long Tail
by Frederick Chong and Gianpaolo Carraro, Microsoft Corp
The first one reviews SaaS from users perspective and is meant for business users. While the second one discusses the underlying architecture & maturity model and is meant for technical architects.