In this post Adam Kinney talks about the new feature of Expresseion Blend RC to load resources at design time.
When building Silverlight and WPF applications that are composited (e.g. using MEF) or written to support UI customization, resources are loaded dynamically at run-time. This is a great solution on the development side, but can be a challenge when working on design tasks. If your design surface can’t find the resources at design-time you are presented with zero styling or worse, XAML parsing errors.
Expression Blend 4 RC provides a new feature to support loading resources at design-time.