Hi Peter,
The TaxonOccurrence standard is not actually a standard. It is part of the vocabulary that was put together two years ago and maps well to DwC.
Meanwhile, as Markus points out, DwC has gone its own way.
The whole TDWG ontology needs some clarification and sorting out and as Bob points out this is now done on a voluntary basis.
I am working on the TaxonName and TaxonConcept vocabularies because I need to use them. I hope I will provide what Markus requests (good return type definitions for GUIDs) but I will only do it for taxon names and concepts because that is all I am working on. I hope to take this to a stage where it is solid, documented and standardizable.
This will entail doing some stuff on the way the TDWG ontology/ vocabulary is presented but really it is not my department to do that alone.
This is very important for TDWG (as Markus points out).
What really needs to happen is for people who need things to happen in the ontology to role up their sleeves and make them happen.
For occurrence stuff I recommend you join forces with John Wieczorek.
All the best,
Roger