The latest Extending and Versioning XML Languages article is up at XML.com. It's incorporated a lot of material that I've blogged about. It now shows comparisons of 3 different version identification strategies with sample xml and schemas for each of these. I added material on Language Questions and Decisions. I added material on the relationship between extension and versioning, and where it is quite unclear whether a component is an extension or a version. I reference Dare's article for the sentry technique for designing compatible schemas.
Some of this material will make into the revised TAG finding that is being worked on over the next few weeks.
The article is also the basis for my talk in a few weeks at xml.com. Any comments or questions are always welcome (except not on my blog because of spammers)