Problem using getElementById with XML Schema

I've recently changed an XML document from using a DTD for validation to
using a Schema.

I specify 'ID' attributes for some of the elements, so that I can locate
them using getElementById, but I can only get this to work when I use a
DTD, not when I use a schema.

My DTD attribute definition is:
    <!ATTLIST SystemManager id ID #REQUIRED>

My Schema attribute definition is:
    <xs:attribute name="id" type="xs:ID" use="required" />

where the xs namespace is declared by:
    <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="unqualified">

I am using Xerces 2.1.0 from C++, but I've also tried using MSXML from
C# without any success.

Any help or suggestions would be much appreciated.

Thanks,

David
-- 
David Miller