Hello Gary Davis,
It is great that you found the cause.
Unfortunately Silverlight is prone to crashing with such obscure errors and there really is no other way to find the cause other than commenting out code and xaml in order to narrow down on it. Also there is no way to know for certain if our controls are causing the crash and if so, how they are causing it.
the Telerik team
Let us know about your Windows Phone 7 application built with RadControls and we will help you promote it. Learn more>>