from Karl
Silverlight 3 application demonstrating Sample Data.
Displaying Sample Data in the WPF and Silverlight Designer for Visual Studio 2010 Beta2 is now very easy with the DesignData markup extension.
The below video is a complete tutorial for using Sample Data in your applications. Please take a view few minutes to view it.
Beta2 Template Information
For Beta2, you need to use these templates to consume sample data in your projects. The reason is the winfx.targets file for was not updated with the sample data build actions in time for Beta2.
These project files have been modified to include the DesignData Build Action for the Sample Data files. All Sample Data files must have their Build Action set to DesignData in the properties window.
These templates will not be required for the final version of Visual Studio 2010.
There are really two templates.
1. <Language> <Platform> Empty Application – this template consists of an empty project file and the added Build Action DesignData.
2. <Language> <Platform> Sample Application – this template consists of a sample application that has sample data files and a UserControl that demonstrates how to consume the sample data. It also includes the added Build Action DesignData.
more here