I finally got around working out how to work with design-time data.
I have same SampleData.xaml when it's feasible and otherwise I've got some DesignTimeXXXPresentationModel which worl great as well.
I know that I can set the SampleData.xaml to DesignData in the Build Action and VS does its magic and leaves it out of the DLL.
I couldn't find any way of achieving that for the design-time class. Is there an easy way of doing that? It seems that people are creating a whole separate project to host all design-time classes and then write .bat files to make sure the assembly doesn't get included in the release.
In another note, I noticed that sometimes the StaticResource defined in the App.xaml will be picked up by the designer in another project and sometimes not.
Is there a way to configure that?