rdf:about=""?

Adobe's specification for XMP [1] tells us that the rdf:about attribute may
be "an empty string ... which means that the XMP is physically local to
the resource being described." I've seen other uses of rdf:about="" being
used to indicate that the metadata shown describes the resource that
contains it. I suppose it's the most abbreviated relative URI possible. Is
it kosher? With or without xml:base being declared? Is this possibility
described at any document in  <http://www.w3.org/tr> www.w3.org/tr? I looked
around and didn't see anything. 
 
(I am aware that XMP uses its own, uhhh, "profile" of RDF, so it may be part
of that.)
 
thanks,
 
Bob 
 
[1] http://partners.adobe.com/asn/tech/xmp/pdf/xmpspecification.pdf
<http://partners.adobe.com/asn/tech/xmp/pdf/xmpspecification.pdf> , page 21