IBM has added a lot more XML support in its most recent version of DB2,
including a new data model specifically for XML. I believe they've
included XQuery support for that. The media have been saying that this
breakthrough might have actually justified calling the new server DB3,
rather than just DB2 version 9.
There's a no-charge version available for download ("DB2 Express-C")
which supports C, C++, Java, .Net, and PHP among others. You'll find a
link to it from IBM's XML DeveloperWorks website (
http://www.ibm.com/xml)
(Statement of bias: I'm an IBMer.)