You can't use AncestorType in Windows Phone 7 Development =( It's not supported by the platform.
This solution works on Silverlight 5 (SL5)
Well I'm an AncestorType fan. Remove innecesary code
{x:Type Grid}
Try this line of code on your XAML:
RelativeSource={RelativeSource AncestorType=Grid, AncestorLevel=1}
Hope it helps =)