Many database vendors are jumping into the XML bandwagon supporting seamless SQL/XML integration . Oracle has a
repertoire of XML-enabled interfaces. So does Sybase, SQL Server etc. The point to note however, is that these tools and frameworks are native and cannot be used as generic( cross-vendor ) solutions.
Having said that, I should point out that it is fairly easy to write your own lightweight class library to format a DB resultset to XML and vice-versa. The book "XML Databases" by Wrox Press ( Author Kevin Williams ) is written specifically to address XML-DB integration and using an XML file itself to represent a small hierarchical database. If you haven't read it, I suggest you check it out!
Good luck,
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Ajith Kallambella M. Sun Certified Programmer for the Java�2 Platform.
IBM Certified Developer - XML and Related Technologies, V1.