Hi Erik Damgaard,
control renders the external pages into an HTML IFRAME
element that is positioned above the Silverlight plugin. Thus, besides hiding it by setting its Visibility property to "Collapsed", there is not much we can do to make it render behind any Silverlight element. This is an inherent restriction of the HtmlPlaceholder
control and although we are looking for possible resolutions, currently we are not aware of ways to work around it.
I hope this is not a showstopper for you.
the Telerik team