How about this page from Microsoft Connect (explaining the DesignData and DesignDataWithDesignTimeCreatableTypes) types. Quoting:
The following describes the two Build Actions for Sample Data files.
Sample data .xaml files must be assigned one of the below Build Actions:
DesignData: Sample data types will be created as faux types. Use this Build Action when the sample data types are not creatable or have read-only properties that you want to defined sample data values for.
DesignDataWithDesignTimeCreatableTypes: Sample data types will be created using the types defined in the sample data file. Use this Build Action when the sample data types are creatable using their default empty constructor.
Not so incredibly exhaustive, but it at least gives a hint. This MSDN walkthrough also gives some ideas. I don't know whether these Build Actions are applicable for non-Silverlight projects also.