The d20-xml project (at Yahoo! Groups) had in mind the idea of specifying a schema for the various sets of d20 data now made available. Unfortunately, I disagree with the way that the project has gone...
As has been discussed on the mailing list for the group, some people insist that the data should be maintained in a human-readable format, while others want the XML to be structured for machine processing. I side with the latter, because my interest is in writing tools for generating d20 output, with human readability of the base data a secondary concern.
It's my belief that XML is not really meant for human consumption. Granted, one of the benefits of XML is the fact that humans can parse it (pretty much), we have XSLT available for turning XML into something else, such as stat blocks for monsters or encounters, or character sheets for characters.
With the way of the world, all commercial products seem to go to the open source world eventually, so the branch project of this xmld20 project will be an open source version of eTools.