PURLs for GO

I think these have been published before but I only just noticed the use of PURLs for references in the Gene Ontology. For example: http://purl.org/obo/owl/GO#GO_0008150 (the gene is not dereferenceable but then that seems okay for current purposes). This is following Recipe 1a from the "Best Practice Recipes for Publishing RDF Vocabularies", although the file sizes are probably too big (as suggested in "Serving Static RDF Files"). It does seem inconsistent with the Banff Manifesto which suggests URLs more like http://purl.org/bm/go:0008150. I know about slashes and hashes but I'm not sure about colons.