Making sure your Silverlight 2 widget will work cross domains

When creating a Silverlight application that will be used as a widget, you need to make sure that it will work in a cross domain scenario. You will certainly serve the widgets from your own domain and the widget will be consumed from other domains, like wordpress.com or blogger.com. You can find this information in msdn and some other blogs but it’s scattered in bits and pieces and not all together, also I struggle with some examples were they set the value of ExternalCallersFromCrossDomain to FullAccess, what is not supported in Silverlight 2 anymore.