Reification

• Cameron Neylon: Why the triple needs to be a quint (or a sext, or...)
• Jeni's Musings: Metadata about RDF triples: reification and Linked Data
• The FreebaseBblog: A Brief Tour of Graphd

In summary: it seems reasonable that every edge of a graph should have a URI, like nodes do, so that you can attach properties to that edge (who made the statement, under what conditions is it valid, confidence levels, an identifier for the previous version, that kind of thing).