I've just posted the Team Foundation schema files for process templates and work item types on Code Gallery: http://code.msdn.microsoft.com/TFSchemas/Wiki/View.aspx?id=25933.
You can drop the schema files in Program Files\Microsoft Visual Studio 10.0\Xml\Schemas and Visual Studio will validate your work item types or process template files using the appropriate schema. For example, if you are adding a field to a work iteim type, Visual Studio helps you by giving you the choice of valid properties and child elements.
With the schemas, Visual Studio also lets you know if you haven't specified all the required properties by using the squiggely underline under the element name.
And it checks for valid strings, too. For example, the refname in the following example uses a dash, which isn't valid.
Without the schema files, Visual Studio can only verify that you have well-formed XML, and you wouldn't get any of these helpful hints.
In some cases, the process tempalte files don't reference the schema files. You can work around this by opening the process template file you want to edit, and then selecting the appropriate tempalte file in the properties dialog.