This is an early comment, but a student from Neumont University has an xml schema provider partially working. There are several schemas, but in a nutshell it is an xml representation of the sql 2003 standard. The project page is located at
http://orm.sf.net (Neumont ORM Architect) ORM not meaning Object Relational Mapping, but rather Object-Role Modeling. The tool itself is an extension to Visual Studio 2005 standard or higher for modeling ORM conceptual models. This xml was designed as a mechanism for generating to the sql standard and overriding per database platform if it's implementation differed from the standard. All of the sql generation tools are xslt based. It would be worth taking a look at for those interested in generating a .nettiers datalayer from an xml file. There are still a few kinks, but impressive from what I've seen in the amount of time that has been put into it.