Schematron is the boss XML schema language your boss has never heard of. Unfortunately it's had some slow times of recent, but it's surged back with a vengeance thanks to honcho Rick Jelliffe with logistical support from Betty Harvey. There's now a working mailing list and a Wiki. Rick says that Schematron is slated to become an ISO Standard next month.
The text for the Final Draft Internation Standard for Schematron has now been approved by multi-national voting. It is copyright ISO, but it is basically identical to the draft at www.schematron.com
The standard is 30 pages. 21 are normative, including schema listings and a characterization of Schematron semantics in predicate logic. Appendixes show how to use other query language bindings (than XSLT1), how to use Schematron as a vocabulary, how to express multi-lingual dignostics, and a simple description of the design requirements for ISO Schematron.
Congrats to Rick. Here's to the most important schema language of them all (yes, I do mean that). I guess I'll have to check Scimitar, Amara's fast Schematron processor for compliance to the updated draft standard.