targetNamespace vs default namespace

Hello,

A customer gave us an xml schema having different default ns and targetNamspace, like that:
<xsd:schema xmlns="ProprietaryNamespace" targetNamespace="AnotherProprietaryNamespace" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
...
    <xsd:complexType name="complexTypeOne">
        <xsd:sequence>
            ...
            <xsd:element name="SubElementWithoutPrefix" type="ElementTypeWithoutPrefix" /> <!-- a line with a problem -->
        </xsd:sequence>
    </xsd:complexType>
    ...
</xsd:schema>

Now we have problem validating values of SubElementWithoutPrefix on different java plattforms.

According to the XML schema specification, every element/attribute without namespace prefix belongs to the default namespace.
On the other hand, unqualified elements defined in an xml schema belong to the schemas targetNamespace. So, there is this contradicition
to be resolved.

When SUNs Java 6 implementation resolves it, SubElementWithoutPrefix belongs to the default namespace. On the other hand,
when IBMs java 5 implementation resolves it, SubElementWithoutPrefix belongs to the targetNamespace.

How is it defined in XML schema specs?

Best Regards,
Tamas