(Update - 2007.10.02: Just realized that there were some serious flaws in the post below regarding publication and form of namespace URIs which I’ve now addressed in a subsequent post here.)
By way of experimenting with a use case for ISO URNs, below is a listing of the document metadata for an arbitrary PDF. (You can judge for yourselves whether the metadata disclosed here is sufficient to describe the document.) Here, the metadata is taken from the information dictionary and from the document metadata stream (XMP packet).
The metadata is expressed in RDF/N3. That may not be a surprise for the XMP packet which is serialized in RDF/XML, as it’s just a hop, skip and a jump to render it as RDF/N3 with properties taken from schema whose namespaces are identified by URI. What may be more unusual is to see the document information dictionary metadata (the “normal” metadata in a PDF) rendered as RDF/N3 since the information dictionary is not nodelled on RDF, not expressed in XML, and not namespaced. Here, in addition to the trusty HTTP URI scheme, I’ve made use of two particular URI schemes: “iso:” URN namespaces, and “data:” URIs.