ARC2 is the PHP library for RDF that we use in Drupal. One thing it gives is the ability to parse a Web page into a triple store that can then be queried by Views using SPARQL Views.
So I definitely thought there already was a parser in ARC2 for microdata. I've gotten in touch with Bengee Nowack, the ARC2 developer and a supporter of both RDFa and microdata, to see whether he knows of one in the works.
Could anyone do research on this? If there isn't one in the works, I don't think it would be too hard to write one for a developer who is familiar with OO.