current status: open. This is a FoafVocab issue, documented as such by linking it from the IssueTracker page.
Problem: The FOAF RDFS file can be fetched from http://xmlns.com/foaf/0.1/index.rdf . (It can also be fetched from the same URL without the index.rdf, when using content negotiation.) The RDFS document contains an owl:Ontology element that is supposed to describe the document itself. Its URI is given as http://xmlns.com/foaf/0.1/ . This does not match the document's URL. An RDF processor cannot tell that the element is supposed to describe the current file.
Solution: Add an xml:base="http://xmlns.com/foaf/0.1/" declaration. This tells RDF processors to use that URI instead of the file's URL for all RDF purposes, removing the mismatch.
Discussion: Thread at http://rdfweb.org/pipermail/rdfweb-dev/2004-September/013678.html
(Summary: RichardCyganiak thinks this is a problem because it could break ontology editors and viewers. BenjaminNowack, DanBrickley and DavidMenendez disagree and say that this is not a problem. But consensus seems to be that adding the xml:base wouldn't hurt either. Dan says he's going to add it in a future site update: http://rdfweb.org/pipermail/rdfweb-dev/2004-September/013690.html)