When you start experimenting with WordProcessingML (or Open XML in general), you definitely must take a look at the following tools:
Word 2007 Content Control Toolkit
This light-weight tool can open any word open xml
document and lists all content controls and available XML parts. It also enables mapping between content controls and custom XML. This all by using the System.IO.Packaging library available in .NET 3.0.
This tool let you look inside a Open XML package, edit XML parts,...
[Thx to Maarten, who recommended me to made use of these]
If you need a lot of information about OpenXML - Download this great book (Open XML: The Markup explained)