First of all some important annoucements!

2 solutions have been released:

Windows based hosting version 4.0 has launched
and
Windows based hosting for Applications 1.0 has launched

WBH for Apps 1.0 is the Microsoft solutions architecture that provides guidance for both ISV's and Hosting infrastructure providers for allowing software services to target long tail business models.

http://www.microsoft.com/serviceproviders/hosters.mspx
http://www.microsoft.com/serviceproviders/solutions/applicationhosting.mspx
http://www.microsoft.com/serviceproviders/solutions/windowsbasedhosting.mspx

Also its important to check out the new SaaS Architecture section on MSDN.

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

Also Fred Chongs Blog provides some excellent insight into SaaS Architectures.

http://blogs.msdn.com/fred_chong/default.aspx