This issue is part of a series of patches which came out of the RDF code sprint. See the list of all RDF patches for Drupal code. Please read State of the RDF in Drupal core after the code sprint which explains the overall approach of the RDF effort in core.
This is the patch to create the core RDF module. It deals with managing the mappings defined by the modules and provides a helper function for generating some of the RDFa attributes for the theme layer. Tests included. It is fairly small in itself. We discussed whether this should be a module or a include library. We went with a module for now but this can still change, especially since maybe half of the RDF code in core will be sprinkled in the other modules defining their RDF mappings (see other separate patches:
#493086: RDF #2: node module
#493094: RDF #3: field module
#493056: Add RDF to the user module