There are (at least) two modules you'll want to look at. One is CCK

http://drupal.org/cck-status

and another is hier

http://cvs.drupal.org/viewcvs/drupal/contributions/sandbox/vauxia/hier/

which comes out of vauxia's experience with Operon

http://www.developerfocus.com/projects/operon/

OK. Thank you very much for your quick answer.
What I see from your links is that :
- Those modules seem to be far from being released
- None of them explicitly deals with RDF

For the second point, is it that RDF approach has been tried but proved to be a dead-end ? Or is it just that it's not been tried ?
Because standard-based approach is really a key *feature* for us : we want to be able to use all the tools that deal with with standard RDF.

--
Sébastien Arbogast

Blog : http://www.sebastien-arbogast.com
The Epseelon Project : http://www.epseelon.org
Projet Phénix : http://phenix.netbios.fr