[trumbjd]

The trick is that the binding mode isn't setup by default for TwoWay. If you set this explicitly it works just fine otherwise changes in the source don't do anything to the actual camera position.

Code:

<cameras:TargetPositionCamera
          Heading="{Binding Heading, Mode=TwoWay}"
          Attitude="{Binding Attitude, Mode=TwoWay}"
          Distance="{Binding Distance, Mode=TwoWay}"
          ShowCameraLight="Always"
          TargetViewport3D="{Binding ElementName=MainViewport}"/>