As stated before your demo works but when put in a real world application it fails.

 

I'm am using Vista Home Premium, FireFox 3.6.6, Silverlight version 4.0.50524.0

 

I cannot show you an example because it is in a secure directory that I can't give access to.

 

I will see if I can make an example that you can have access to that duplicates the problem.