Norm Walsh has just issued DocBook NG: The “PTO” Release.
Changes are primarily around XInclude support and the section content models.
This may very well be the final NG release prior to the DocBook V5.0 alpha release we are planning for August.
Please try this out, and report any problems ASAP! Norm has been gracious enough to also produce a Simplified NG version of "PTO", available at: http://docbook.org/docbook-ng/simple/pto/index.html :-)
See also: DocBook