I posted earlier this year on the support for custom defined schema in wordprocessingML via the new content controls functionality. The key reason for opening the file formats was to make Office a more valuable platform for solution builders. We wanted folks to have the ability to take Office documents and plug them into new and existing business processes. All the work we've done in Ecma has helped to ensure that people have the necessary information for implementing the new file formats, but that isn't enough. The Office Open XML specification defines the XML for Office documents, but it doesn't define the types of structures that our customers use for their own business data. That's where the support for custom defined schema comes into play. It allows people to take their industry specific data structures (either their own, or one defined by someone else), and apply that structure to their documents so they have that additional semantic meaning. I'd promised to post more information on this but that fell through the cracks (sorry).
Tristan Davis from the Word team is now blogging on content controls and he'll be able to cover it in much greater detail than I could. You should check out his first post up on the Word blog: http://blogs.msdn.com/microsoft_office_word/archive/2006/10/23/control-yourself.aspx