I’ve just release version 0.1.0 of the RDF::Microdata gem (rubygems, github) for the RDF.rb suite. This version contains only a reader (parser). Writing is not supported at this point.

Use it pretty much like any other RDF.rb reader:

graph = RDF::Graph.load("etc/foaf.html", :format => :microdata)

Feedback either to me, or public-rdf-ruby.