Hi, I just want to make a note about a new "smart untriangulation tool" called Quadrangler: See a short video on Youtube and further info and download on http://c4dplugin.com.
The tool is free for customers of one of the commercial products.
UPDATE: Version 1.2 is released and comes with an Unsubdivider. It's not free anymore, but a quite low price. The video link has changed: