I've written a new Office Talk column on creating form letters in Word 2007 and Word 2010 by using bookmarks and Office Open XML. The column, titled Creating Form Letters in Word by Using Bookmarks and Office Open XML Files is available at:
In the column I describe how to create a Visual Studio console application in both C# and Visual Basic .NET. The project uses automation to retrieve addresses from an Excel workbook, and populates a sequence of bookmarks in a Word document (created from a template) which has been opened as an Office Open XML package. One form letter is created for each address retrieved. Check it out.