Microsoft Patterns & Practices have several nice application architecture pocket-guides about RIA & Silverlight and Web applications.

RIA-Silverlight pocket guide:

http://www.codeplex.com/AppArch/Release/ProjectReleases.aspx?ReleaseId=19799
http://blogs.msdn.com/jmeier/archive/2008/11/24/ria-architecture-pocket-guide.aspx

Web pocket guide:

http://www.codeplex.com/AppArch/Release/ProjectReleases.aspx?ReleaseId=19802
http://blogs.msdn.com/jmeier/archive/2008/11/24/web-application-architecture-pocket-guide.aspx

All P&P Pocket guides:

http://www.codeplex.com/AppArch/Wiki/View.aspx?title=Pocket%20Guides&referringTitle=Home