A little while ago some members of our team experimented various ways to reduce the Relational<->Objects<->XML (ROX) impedance mismatch by adding concepts and operators from the relational and XML (specifically W3C XML Schema) world into an object oriented programming language. This effort was spear headed by a number of smart folks on our team including Erik Meijer, Matt Warren, Chris Lovett and a bunch of others all led by William Adams. The object oriented programming language which was used as a base for extension was C#. The new language was once called X# but eventually became known as Xen.
Erik Meijer presented Xen at XML 2003 and I blogged about his presentation after the conference. There have also been two papers published about the ideas behind Xen; Programming with Rectangles, Triangles, and Circles and Unifying Tables, Objects and Documents. It's a new year and the folks working on Xen have moved on to other endeavors related to future versions of Visual Studio and the .NET Framework.
However Xen is not lost. It is now part of the Microsoft Research project, Cw (pronounced C-Omega). Even better you can download a preview of the Cw compiler from the Microsoft Research downloads page