The System.ServiceModel.Channels.HttpResponseMessageProperty.SuppressPreamble property (true if the message preamble is suppressed; otherwise, false; this property is false by default) is part of the .Net Framework 4.0. A QFE adds this property to .Net 3.0. The QFE is available here:
http://hotfix.partners.extranet.microsoft.com/FixDetails.aspx?fixid=405608
This property is documented here:
http://msdn.microsoft.com/en-us/library/system.servicemodel.channels.httpresponsemessageproperty.suppresspreamble.aspx
Ed Price MSFT edited Revision 3. Comment: Updated title case.
Patrick D. Fletcher edited Revision 1. Comment: Added link to 4.0 documentation for new property