Our Microsoft patterns & practices Application Architecture Guide is now linked from the ASP.NET Home.  Many thanks to Joe Stagner for making this happen.

You can read Scott Guthrie's foreword for the guide online. Here's the key features of the guide at a glance:

• a canonical application frame
• a set of canonical application types (Web, RIA, mobile ... etc.)
• an architecture frame (caching, data access, exception management ... etc.)
• quality attributes (security, performance, manageability, ... etc.)
• principles, patterns, and practices
• technologies and capabilities

From an ASP.NET standpoint, you get design level guidance for Web applications, and there's a technology matrix at the end of the guide.  Think of the guide as a map of the terrain to help you find the hot spots for your architecture and design.