Software as a Service is a new delivery model where companies pay not for owning the software itself but for using it. In the SaaS scenario, a software provider is responsible for its availability (maintenance, scalability, disaster recovery, etc.). That’s what the companies pay for.
Check out the architecture stratagies http://msdn.microsoft.com/architecture/saas/default.aspx?pull=/library/en-us/dnbda/html/ArchStratCtchLngTail.asp
Tips on software as a service : Mike Obrien's blog
http://blog.mike-obrien.net/PermaLink,guid,d4255778-668c-4922-ba22-a78335e3b8a0.aspx
Also take a look at this article on multi tenant data architecture at the microsoft solution architecture center
http://msdn.microsoft.com/architecture/default.aspx?pull=/library/en-us/dnbda/html/MlttntDA.asp
Fred Chong's blog on SAAS http://blogs.msdn.com/fred_chong/
Gianpaolo's blog http://blogs.msdn.com/gianpaolo/

Update(04/10/06) : Akila Manian delivered a webcast on Shift in paradigm from business solution to business services which can be listened on demand at -
https://msevents.microsoft.com/CUI/Register.aspx?culture=en-IN&EventID=1032309843&CountryCode=IN&IsRedirect=false