Same question as the one archived here:
http://social.msdn.microsoft.com/Forums/expression/en-US/de7f74e7-eea0-4e4b-87ee-659b716abd75/datepicker-nullable-date
I'm familiar with using value converters to present a bound parameter value nicely in the UI but can't see how one could be used with the XAML DatePicker control to prevent it showing a date by default.