The openid plug-in for WordPress is now stable (I made the experiments on svn versions), so here’s the piece of code that retrieves profiles when creating user account – tested with v2.1.2 of wp-openid.
Unpack the tgz in your the wp-openid folder, and check readme-foaf.txt for instructions. This version also features a function you can add in your templates to show the FOAF icon and a link to the profile in comments, and a way to cron the autodiscovery feature for profiles updates (Edit: The cron now tries to find FOAF files from user homepages, see comments).
I’ll maintain this hack for next updates of wp-openid (unless it’s in the plug-in itself). Nexts steps will be to retrieve people URI (more than just the profile, which should be easy if people use the foaf:openid property in their profile) and use these data in the SIOC and FOAF exports for WordPress. It will be one more way to provide a RDF-ized social graph, with reference to existing profiles and URIs.