I've stoumbled over this Exception when I installed WSE 2.0 SP2 on my machine
and called Wse WEB Service.
Non-Wse WEB Service worked fine.

The bug could be reproduced like this:

1.
    DataColumn[] arrKey = new DataColumn[1];;
    arrKey[0] = ds.Tables["Request"].Columns["RequestID"];
    ds.Tables["Request"].PrimaryKey = arrKey;

    DataColumn[] arrKey2 = new DataColumn[1];;
    arrKey2[0] = ds.Tables["Package"].Columns["PackageID"];
    ds.Tables["Package"].PrimaryKey = arrKey2;
    ds.Tables["Package"].Columns[arrKey2[0].ColumnName].AutoIncrement = true;
    ds.Tables["Package"].Columns[arrKey2[0].ColumnName].AutoIncrementSeed = -1;
    ds.Tables["Package"].Columns[arrKey2[0].ColumnName].AutoIncrementStep = -1;

...

In this case the exception would be:

There is an error in XML document (1, 4321).  --> The
'http://tempuri.org/:Request_Constraint1' identity constraint is not
declared.  An error occurred at , (1, 3905)."}

When I saved ds in XML file it looked like:

<?xml version="1.0" standalone="yes" ?>
- <NewDataSet>
- <xs:schema id="NewDataSet" xmlns=""
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
- <xs:element name="NewDataSet" msdata:IsDataSet="true" msdata:Locale="en-CA">
- <xs:complexType>
- <xs:choice maxOccurs="unbounded">
- <xs:element name="Request">
- <xs:complexType>
- <xs:sequence>
 <xs:element name="RequestID" type="xs:string" />
 </xs:sequence>
 </xs:complexType>
 </xs:element>
- <xs:element name="Package">
- <xs:complexType>
- <xs:sequence>
 <xs:element name="PackageID" msdata:AutoIncrement="true"
msdata:AutoIncrementSeed="-1" msdata:AutoIncrementStep="-1" type="xs:int" />
 <xs:element name="RequestID" type="xs:string" minOccurs="0" />
 <xs:element name="PackageName" type="xs:string" minOccurs="0" />
 <xs:element name="PackageDescription" type="xs:string" minOccurs="0" />
 <xs:element name="DueDate" type="xs:dateTime" minOccurs="0" />
 <xs:element name="PackageTypeID" type="xs:int" minOccurs="0" />
 <xs:element name="UserID" type="xs:int" minOccurs="0" />
 <xs:element name="NoOfConsultees" type="xs:int" minOccurs="0" />
 <xs:element name="NoOfPages" type="xs:int" minOccurs="0" />
 <xs:element name="Timestamp" type="xs:base64Binary" minOccurs="0" />
 <xs:element name="ApproveDate" type="xs:dateTime" minOccurs="0" />
 </xs:sequence>
 </xs:complexType>
 </xs:element>
- <xs:element name="Consultee">
- <xs:complexType>
- <xs:sequence>
 <xs:element name="PackageID" type="xs:int" />
 <xs:element name="UserID" type="xs:int" />
 <xs:element name="IsHost" type="xs:boolean" minOccurs="0" />
 <xs:element name="DisposedDate" type="xs:dateTime" minOccurs="0" />
 <xs:element name="PackageStatusTypeID" type="xs:int" minOccurs="0" />
 <xs:element name="Timestamp" type="xs:base64Binary" minOccurs="0" />
 <xs:element name="PackageViewRightsTypeID" type="xs:int" minOccurs="0" />
 <xs:element name="PackageCopyRightsTypeID" type="xs:int" minOccurs="0" />
 </xs:sequence>
 </xs:complexType>
 </xs:element>
- <xs:element name="PackageStatusType">
- <xs:complexType>
- <xs:sequence>
 <xs:element name="PackageStatusTypeID" type="xs:int" />
 <xs:element name="PackageStatusTypeName" type="xs:string" minOccurs="0" />
 </xs:sequence>
 </xs:complexType>
 </xs:element>
- <xs:element name="PackageApproval">
- <xs:complexType>
- <xs:sequence>
 <xs:element name="PackageApproveTypeID" type="xs:int" minOccurs="0" />
 <xs:element name="UserID" type="xs:int" />
 <xs:element name="PackageID" type="xs:int" />
 </xs:sequence>
 </xs:complexType>
 </xs:element>
 </xs:choice>
 </xs:complexType>
- <xs:unique name="Constraint1" msdata:PrimaryKey="true">
 <xs:selector xpath=".//Request" />
 <xs:field xpath="RequestID" />
 </xs:unique>
- <xs:unique name="Package_Constraint1" msdata:ConstraintName="Constraint1"
msdata:PrimaryKey="true">
 <xs:selector xpath=".//Package" />
 <xs:field xpath="PackageID" />
 </xs:unique>
- <xs:unique name="Consultee_Constraint1"
msdata:ConstraintName="Constraint1" msdata:PrimaryKey="true">
 <xs:selector xpath=".//Consultee" />
 <xs:field xpath="PackageID" />
 <xs:field xpath="UserID" />
 </xs:unique>
- <xs:unique name="PackageStatusType_Constraint1"
msdata:ConstraintName="Constraint1" msdata:PrimaryKey="true">
 <xs:selector xpath=".//PackageStatusType" />
 <xs:field xpath="PackageStatusTypeID" />
 </xs:unique>
- <xs:unique name="PackageApproval_Constraint1"
msdata:ConstraintName="Constraint1" msdata:PrimaryKey="true">
 <xs:selector xpath=".//PackageApproval" />
 <xs:field xpath="PackageID" />
 <xs:field xpath="UserID" />
 </xs:unique>
- <xs:keyref name="R2" refer="Package_Constraint1">
 <xs:selector xpath=".//Consultee" />
 <xs:field xpath="PackageID" />
 </xs:keyref>
- <xs:keyref name="R3" refer="PackageStatusType_Constraint1">
 <xs:selector xpath=".//Consultee" />
 <xs:field xpath="PackageStatusTypeID" />
 </xs:keyref>
- <xs:keyref name="R1" refer="Constraint1">
 <xs:selector xpath=".//Package" />
 <xs:field xpath="RequestID" />
 </xs:keyref>
 </xs:element>
 </xs:schema>
- <Request>
 <RequestID>---</RequestID>
 </Request>
- <Package>
 <PackageID>-1</PackageID>
 <RequestID>---</RequestID>
 <PackageName>P3</PackageName>
 <PackageDescription />
 <DueDate>2005-02-21T00:00:00.0000000-05:00</DueDate>
 <PackageTypeID>1</PackageTypeID>
 <UserID>1</UserID>
 </Package>
 </NewDataSet>

2.

    DataColumn[] arrKey2 = new DataColumn[1];;
    arrKey2[0] = ds.Tables["Package"].Columns["PackageID"];
    ds.Tables["Package"].PrimaryKey = arrKey2;
    ds.Tables["Package"].Columns[arrKey2[0].ColumnName].AutoIncrement = true;
    ds.Tables["Package"].Columns[arrKey2[0].ColumnName].AutoIncrementSeed = -1;
    ds.Tables["Package"].Columns[arrKey2[0].ColumnName].AutoIncrementStep = -1;

    DataColumn[] arrKey = new DataColumn[1];;
    arrKey[0] = ds.Tables["Request"].Columns["RequestID"];
    ds.Tables["Request"].PrimaryKey = arrKey;

...

The XML DataSet representation was the same except Package_Constratint1 was
not declared and the following exception was thrown:

There is an error in XML document (1, 4321).  --> The
'http://tempuri.org/:Package_Constraint1' identity constraint is not
declared.  An error occurred at , (1, 3905)."}

It looks like XML parser always skips Constraint Definition for the first
table and WSE2.0 just reports the error and throws the exception.