RDF

Is it possible to use the rdf api to automatically tag content written in forums or blogs? If so, can anyone give me some pointers in getting started with this. I have looked at the Calais module which looks really good but I am looking for something more 'site wide' i.e. I want to rdf all of my sites content internally.

Also, with alot of the semantic functionality going to be built into to Drupal 7, what would be the wisdon of going ahead and building a site using Drupal 6 and taking advantage of the contributed modules (rdf api etc) as opposed to building it using Drupal 7 which is still unstable?

Any advice at all would really appreciated.