With the increasing use of XML for business documents and data transfers, interest is growing in searchable repositories for XML-based data and documents. The XPath and XQuery standards are stabilised, and capable products are now available.
The major database engines have provided their own distinct approaches for retrofitting storage of XML content, including mapping of document elements to RDB table columns and exposing for structured query and update. Unfortunately there are no standard approaches - each player is doing it differently.
There is also growing use of XML as the method for packaging client/server queries and responses.
If you need Relational / XML Integration, or a pure XML database, SMH Systems are the XML and Relational Database experts who can provide the advice you need, and develop Proof-of-Concepts.