See the article on the International Financial Law Blog http://lawprofessors.typepad.com/intfinlaw/2014/07/w8ben-w8bene-w8eci-w8exp-and-w8imy-withholding-agent-instructions-released.html
Free FATCA Lexis chapter download at http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2457671