There is a new paper Implementing an RDF Triple Store using an ORDBMS at the Virtuoso wiki.
This paper summarizes how we have extended Virtuoso's SQL and database engine to better accommodate storing RDF triples and optimizing queries of RDF data. This is the first of a series. The next will concern mapping relational databases onto RDF ontologies for SPARQL access.
This paper concerns the next Virtuoso Open Source release, to be available for download a few days from this posting.