RDF and other monkey wrenches has caused responses such as: We the Monkeys, YARD: Yet another RDF diss and RDF is a monkey wrench?.
In some ways I think Sean has missed the point. Sean says we should write our XML vocabularies and convert it to RDF. But what about using tools to create an RDF ontology or taxonomy and convert down to XML? Tools like Protégé or KAON's ontology modeller all allow you to model your information without realising you're using RDF. RDF, being more abstract, has the ability to be superior to XML. The UI can be just like any other XML tool around or better (more flexibility, reuse, adaptive, etc.). You don't have to understand the RDF model. However, in the meantime I see nothing wrong with existing tools and applications producing XML that can be converted to a generic RDF ontology. Indeed, if Semantic Web Services is to take off this will be a neccessity. See RDFT.
Please don't make the same mistakes (like NULL) as SQL - that is preserve the model when implementing.
To quote Fabian on XML: "The model they did come up with is the same hierarchic model which we discarded 30 years ago and replaced with SQL, because it was too complex, inflexible and lacked rigor." See also The Data Exchange Tail - Part 2.