I was very surprised to see that the following was not included among the various ways to convert a docx document to doc format in the July 2010 issue of MacWorld. [crarko adds: They do mention using TextEdit to do the conversion there, but not textutil.]
If you are using Leopard or Snow Leopard, open a Terminal window, navigate to the location of the .docx file you wish to convert and type the following:
textutil -convert doc myFile.docx
Where myFile.docx is the file you need to convert. It's also possible to just drag your document in the Finder and drop it on the Terminal window to get the correct full path to it. The operation will output myFile.doc in the same folder as the original after the conversion.
[crarko adds: I tested this, and it works as described. We've covered textutil before but not this particular conversion.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100616174955678