I will post the answer I sent to your support ticket here as well:
The RadHtmlPlaceholder control is essentially an 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.
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