<p>I'm interested in using Office 2007 to convert between the pre-2007 binary formats (.doc, .xls, .ppt) and the new Office Open XML formats (.docx, .xlsx, .pptx)</p>

<p>How would I do this? I'd like to write a simple command line app that takes in two filenames (input and output) and perhaps the source and/or destination types, and performs the conversion.</p>

Microsoft has a page which gives several examples of writing scripts to "drive" MS Word.
One such example
shows how to convert from a Word document to HTML.  By changing the last parameter to any values listed
here
, you can get the output in different formats.