AlwaysOn  has published a great article which outlines Microsoft's SaaS strategy and resources for ISVs that are thinking to build SaaS application and OnDemand architecture. You can find the article here

This article seeks to create a dialog about what the right platform for SaaS is – and to demonstrate through our successful startups that we do, in fact, have what it takes to help companies grow scalable, highly usable SaaS/online services businesses.

Successful SaaS companies built on the Microsoft platform:
• MySpace—built on Microsoft from the start—100 million users, more traffic than Google, 2nd to Yahoo
• NewsGator--receives nearly 10 million API requests on an average day and nearly a million users are signed up for its consumer
products
• ExactTarget—email marketing company--more than 5,000 requests per minute and over 200 million new records added every day
• EchoPass-- delivering end-to-end customer contact solutions for clients at a fraction of the cost and with far greater flexibility
than traditional solutions
• Lathian-- offers life sciences companies an integrated, full-service range of online sales and marketing tools that help them
improve customer relationships
• Inrix-- By leveraging the strength of Microsoft’s server platform, Inrix processes and analyzes billions of traffic-related data
points to “literally predict the future”. (IP Ventures deal)
• Digipede-- providing a key technology for anyone providing SaaS on the Microsoft platform: distributed grid computing for greater
application performance