I just learned this from http://amatterofdegree.typepad.com/a_matter_of_degree/2008/05/how-to-get-shar.html

When you create a custom Content Type for a document template, and add custom metadata fields to it, you can insert the metadata values into your Word document as Quick Parts.

Several elements need to be in place to make this work smoothly. In my example:

1. I created a Content Type for a NonDisclosure Agreement (NDA)
2. I created a Site Column for Opportunity (i.e., the client name)
3. I added the site column to the NDA content type
4. I added the content type to one of the document libraries on my SharePoint site
5. I created a new NDA from the “New” menu on the document library and
6. IMPORTANT! I saved the document back into that document library. (The Properties Quick Part will appear grayed-out until the document is saved.)

Then, with the document still open in Word, I was able to choose Insert -> Quick Parts from the ribbon, and my “Opportunity” field is available.

If the metadata field is already filled in for the document, you will see the actual data appear in your document (just as when you add the a Date field and today’s date appears).

If the metadata field is empty, you can type the information into the document field, and you will see it will appear in the Properties (Server) window in the ribbon..

Lots of potential for bringing your paper forms online and ensuring better metadata compliance!

Here’s a nice post from the Microsoft Word team on the subject – they refer to it as XML Mapping.