Jonathan Borden wrote:

When I say that the rddl:nature of http://example.org/foo.xsd is "XML Schema", this is intended to assert that it is reasonable to assume that http://example.org/foo.xsd ought comply with the "XML Schema" specification i.e. validate as an "XML Schema".

I believe this to be sufficiently asserted by xlink:role="http://www.w3.org/2001/XMLSchema"

What I *don't* want to say is that <http://example.org/foo.xsd> is a member of the XML Schema namespace. 

Good. xlink:role="http://www.w3.org/2001/XMLSchema" does not say that.

In fact, I'm not sure anything would. URLs and documents are not generally considered to be members of a namespace. The document at http://example.org/foo.xsd could say that the root element is a member of the namespace with a xmlns="http://www.w3.org/2001/XMLSchema" attribute; but that's a very different thing.

Using <http://www.w3.org/2001/XMLSchema> as the URI for the nature of "XML Schema" creates this ambiguity for ***software agents***. 

In practice XML software agents are indeed smart enough to distinguish between xlink:role="http://www.w3.org/2001/XMLSchema" and xmlns="http://www.w3.org/2001/XMLSchema" and even xlink:href="http://www.w3.org/2001/XMLSchema". I don't think there's any ambiguity here we need to worry about.