303 and concept URIs - towards GUIDs for all

I've bookmarked some stuff relevant to the issue of what HTTP URIs identify. This concerns whether a URI identifiers something in the real world (or a concept), or a document. The "303" refers to the HTTP code returned if the URI is a concept, not a document. The whole topic is a mess, but I need to get to grips with it as I'm working on a service to assign URIs to just about any biologically interesting object. Sigh.