Just posted, "Applying the relational model to SPARQL" to the DAWG comments list. It describes the work done adding SPARQL support (or small bits of it anyway) to JRDF. A HTML version to follow.
Direct link (PDF ~500K).
BTW, if anyone notices any grammatical errors please let me know and I'll fix them right up. I get to the point where I can't read what I've written anymore so there's bound to be a few errors.
Update: I've updated it with some small typos fixed, formatting and the example relations (some conversions to S1 -> Supplier1 that I missed) were fixed up.
Update 2: The HTML version is now available. Many thanks to Eric Prud'hommeaux for the work he did converting the original PDF.