When I first watched the Ruby on Rails migration screencast the first thing I thought was, wow, that is exactly what I've dreamed of having to solve the problem of versioning an Xml API.
Ok I lie, the first thing I thought was, holy crap, you mean I wouldn't have to maintain a bunch of one way SQL scripts for migrating a database.
I have no clue what it would take to get Migrations working over an Xml document format(s). Maybe ActiveRecord could be made to work over Xml, or maybe the ideas behind Migrations could be applied some other way. Eventually I want to dig into that. But maybe the lazyweb will beat me to it.