I had a Dahon XX for awhile that came stock with a CF seat post. Beautiful and light weight. CF is not a problem if properly engineered and manufactured, and appropriate for the use. Before getting sidetracked by life, was looking for a CF post for the Birdy. Several outfits were selling them on ebay, and also ran across Asian bike shops selling them, mostly in Japan. I would have no problem with using a CF seat post if the quality was known. I would avoid trying to use a non folder designed seat post. The length and diameter are quite a bit larger in most cases.

I think with folders that trying for less weight isn't so important insofar as riding is concerned, it can make a big difference when lugging it around and stuffing it in public transport during multi-modal use. If you've ever had to run between trains at the Tokyo main station, you probably catch my drift.