When exposing a BizTalk orchestration as a web service, I ran into a basic problem that was not obvious to me at first. When try to invoke the web service I got the exception: "SoapException: Internal SOAP Processing Failure". The basic BizTalk help offered no insight into this problem. The answer can be found on MSDN however. According to MSDN:
Check the identity under which the application pool is running. Is it a member of the IIS_WPG and BizTalk Isolated Host Users groups? These two groups give the ASMX page (the proxy Web service that the BizTalk Web Service Publishing wizard generated) the appropriate rights to insert the SOAP request message into the BizTalk MessageBox in SQL Server that will then launch the orchestration. Symptom may be "SoapException: Internal SOAP Processing Failure".
This fixed the problem for me.