I have a new in-depth article published online at DZone called “Using Portable Class Libraries with Windows 8 / Windows Phone 8 MVVM”. If you're looking to get up to speed quickly with PCL and how they may be used with the MVVM pattern then this is an article that you don’t want to miss.
See below for more information and a link to the online article.
Portable Class Libraries (PCL) support the cross-platform development of .NET Framework applications. With Portable Class Libraries you can write portable assemblies that share code with Windows 8 Windows Phone Xbox 360 and other .NET applications. If you choose not to use them then you must target a single application type and rework the class library for other application types as needed. PCLs save this work by allowing you to build assemblies that are shared across different platforms. In this article we are going to use the MVVM (Model-View-View-Model) pattern with Portable Class Libraries that target Windows 8 apps and more! Let’s get started.