The holy grail of .NET developers and architects has been updated for October 2009. Application Architecture for .NET: Designing Applications and Services from 2002 has finally been replaced by a new version.
Microsoft Application Architecture Guide, 2nd Edition
The new version is available for viewing on the patterns and practices website here: http://msdn.microsoft.com/en-us/library/dd673617.aspx and is definitely recommended reading if your are designing or coding an application in .NET