Whenever I'm developing a non-desktop Windows 8 application I prefer having as much business logic in portable class libraries as possible. The test project can then be a standard .NET class library, allowing mocking frameworks and other helper libraries to be used which are not available elsewhere. Having a dependency on a native platform specific library, such as SQLite, can still complicate things a bit.
Portable class libraries have recently become so ubiquitous, that it's easy to forget they don't exist all that long. As long as you're using Visual Studio 2012 or later and targeting .NET 4.5, this is not really important. But once you start targeting .NET 4, you should better keep that in mind.
On Monday our local Microsoft subsidiary organized Visual Studio 2012 bootcamp as a preconference to Bleeding Edge 2012. For my talk I selected 3 new features in .NET Framework 4.5 that excite me most.
Yesterday two local development user groups organized a common Visual Studio 2012 Community Launch event. I was one of the four MVPs speaking there. Each one of use prepared a talk on his favorite feature. My topic where Portable Class Libraries.