Vista - Soap Body different for different calls

Hi all,

I have a WCF service that accepts a typed dataset. The signature looks
like this:

void ActivatePersonalAccount(ref AccountMaintenanceSDSC sds);

I am trying to invoke this both from a .net client and from BizTalk.
The Soap Body element looks like below for each call. The differences
are that for the call from the .net client application, 1. The Body
has a namespace and 2. On the third line, the Element name is the name
of the parameter (sds) in the method signature rather than the name of
the type

(consume from .net client application)
<s:Body xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<ActivatePersonalAccount xmlns=" ...
<sds msdata:SchemaSeria...

(consume from BizTalk)
<s:Body>
<ActivatePersonalAccount xmlns=" ...
<AccountMaintenanceSDSC msdata:SchemaSeria...

My problem is that when BizTalk calls the method, the dataset instance
inside the method is null. What could be the problem? Thanks!