Only a few people (including me until a few weeks ago) know that Expression Blend supports so called “Design Time Attributes”. These properties are very valuable to get “Design Time Data” into Blend. Without those example data designing DataTemplates is cumbersome and no fun. Those Design Time Attributes are not documented at all and I found out about them only by chance, reading some blogs. It seems that they are officially introduced with VS 2010 and documented there. Nevertheless they are fully functional with VS 2008 because they are ignored by the compiler and only used by design tools like blend.
All Design Time Attributes live in the d: namespace: