When designing custom exceptions for .NET, MSDN provides these guidelines. In particular, the guidelines state that a custom exception:
Basically, ignore that guidance - that is for full .NET, and does not apply to portable class library projects. Indeed, if we look at (say) Silverlight (which includes WP7) we see:
[ClassInterfaceAttribute(ClassInterfaceType.None)]
[ComVisibleAttribute(true)]
public class Exception
Frankly, the main consumer of that requirement was remoting... and that is not in huge demand now.