I noticed on planet KDE this post on desktop search. It mentioned not using xattr for metadata because some filesystems don't support it. I'd say that most filesystems don't, iso filesystems, NFS (depending on setup), http, ftp and xemacs don't. The simple solution to all this in libferris is to virtualise xattr just like the filesytem itself is virtual. So you store xattr in RDF when the underlying filesystem doesn't allow it.
I should also highlight that the tagging mentioned in the post referenced by the above post is already available and usable with libferris :) You can attach arbitrary metadata to virtual filesystem objects, index them and search based on that metadata. Indexing can be done in many formats, lucene, postgresql, RDF using redland (db4, sqlite, postgresql) or on an LDAP server.