The definition of a SQL-like query language for XML has been a long time coming. XQuery is starting to gel, though. You can read about it here:
XQuery: A flexible query language for XML (Dr. Dobb's Journal) [ IBM DeveloperWorks: XML News]
And if you're interested, you can try it out on the OpenLink Software site. The implementation is part of OpenLink's new version of Virtuoso, which they call a universal data integration server.
Former URL: http://weblog.infoworld.com/udell/2002/03/18.html#a147