Using XmlSchemaSet to validate WordprocessingML
Wouter Van Vugt has posted some useful information about the issues and options for validating XML schemas in .NET applications: Validating Open XML documents with System.Xml - A WordProcessingML sample. If you're writing code that works with schemas that have circular references to one another, as occurs in some of the Open XML schemas, you'll want to check it out. Thanks for explaining this, Wouter!
On a related note, Wouter has a great series of articles about working with WordprocessingML over on OpenXmlDeveloper.org.