Matt runs through the whole range of options from base64, Soap Messages with Attachments, WS-Attachments (a la WSE1.0 and 2.0) and MTOM. Good read for a picture of where this is, what to use when and where it's ultimately going. Link is here http://msdn.microsoft.com/webservices/default.aspx?pull=/library/en-us/dnwebsrv/html/opaquedata.as
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