Increasing the bounding volume is required.  If you disable the line that sets boundingVolumeExtend in the sandcastle example and zoom towards ground level looking down you can end up with something like this...
![snap](https://f.cloud.github.com/assets/484870/1547253/af5a31ee-4d99-11e3-854a-6f52de739d36.jpg)

Feel free to move it if there's a better place for it.