Following on from the previous post, as Howison and Goodrum note, Adobe provides XMP as a way to store metadata in files, such as PDFs. XMP supports RDF and namespaces, which means widely used bibliographic standards such as Dublin Core and PRISM can be embedded in a PDF, so the article doesn't become separated from its metadata. Adobe provides a developers kit under a BSD license.
The vision of managing digital papers being as easy as managing digital music is really compelling. Imagine auto populating bibliographic management software simply by adding the PDF! Note also that the Creative Commons supports XMP.
