- 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).
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It's quite funny to see this issue coming up again all over the place, I vaguely remember ages ago on rdf-dev or similar Seth Russell and/or Sean Palmer putting up quite good arguments for why it should be pents...
But anyhow, where refactoring to n-ary relations is inconvenient, named graphs seems like the current best practice - you can always stack them up. (With RDF reification being quietly deprecated).
It's always going to be a trade-off between expressiveness and computational complexity - easiest to start with the easier cases...