Chris Klug discusses Merged Dictionaries in Silvleright 3 and how this could help you if your XAML is getting overloaded with all the things you want to drag from application to application.
So what do MergedDictionaries offer? Well, they make it possible for you to store resources in external files. These files are then either embedded in the application as Content or as Resources. This means that we can spread out or resources in several files and just reference them in our controls, or possibly in our applications resources. This makes our Xaml files shorter and easier to read. We only declare our resources in one place and can reuse them in a good way. We can even store them in external assemblies and reuse them across several projects.