I've got a client who doesn't fundamentally understand SOAP, much less XML.  They are insisting on a specific prefix for elements in a SOAP Response we are sending to them.

They want:

       <soap:body>
             <samlp:Response xmlns:samlp="thewholedurnthing">  
                  <samlp:somestuff>

I'm producing via an ASMX :

         <soap:body>
             <Response xmlns ="thewholedurnthing" xmlns:samlp="thewholedurnthing">
                  <samlp:somestuff>

and they are not digging it.  Certainly the prefix doesn't matter as the InfoSet is semantically the same.  I suspect the client (who is using Java) is manually <gasp> parsing the Response.

I'm hoping to cover this experience in an upcoming PADNUG talk now tenatively entitled "How NOT to do Web Services: Is ritual suicide the only answer?"

I messed around with the obvious XmlRoot and XmlType attributes and observed they were being wholly ignored for the Root Element of the resulting SOAP payload.  I talked to Christian Weyer and Herr Vasters who confirmed that it is 99.9% not possible via simple attributes. 

So, taking their advice I will write a quickie SoapExtension, probably something along the lines of:
     XmlRootForcePrefix(prefix="samlp", namespace="thewholedurnthing").