IBM DB2 and XForms
IBM DB2 version 9 (also known as the "PureXML" release) supports both the XQuery language as well as an XML datatype. This makes it an ideal candidate for storing and retreiving your XForms instance data.
Unfortunately as of March of 2007 there is no pre-written REST interface to DB2 or a web-based administration tool for setting up and configuring DB2 XML collections.. To save your form data you will have to write some Java middleware to take the data in an HTTP POST and save it to a table in DB2 that has an XML datatype.
Here is an example from the IBM DeveloperWorks web on how to use Java to send XML data to DB2:
The good news is that once the data has been saved into DB2 you can query it using standard XQuery tools.