LINQ to XSD is a soon-to-be-released extension to LINQ that code-generates strongly typed classes for XML data based on an XML schema. You can download LINQ to XML or download the alpha bits for LINQ to XSD, which will allow you to write LINQ queries against XML data. Adding restrictions to the XML data via the XSD file provides the same good benefit that you get from adding constraints to databases—it facilitates data integrity.
Paul Kimmel is an architect for EDS and the cofounder of the Greater Lansing Area .NET Users Group (glugnet.org, Flint and East Lansing). Look for his book LINQ Unleashed: for C# and his upcoming book Teach Yourself the ADO.NET Entity Framework in 24 Hours (Sams, spring 2009). If you have a programming question, you can contact Paul at email@example.com.