I just published this WCF adapter video examining, in detail, the flow of an incoming WCF message into a BizTalk receive location that is configured to use a WCF receive adapter. See how this message is converted by the WCF adapter into a BizTalk message and stored in the Messagebox database.
http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=c94f6263-3cd4-43b5-b038-5737a8d0d219