Jim is correct, if you use the search function you will see where this very topic was hashed out and beaten up pretty good a few months back.
I believe every company has to install vacuum breakers on the flush line, which will prevent contaminants from reaching the potable water supply.
As long as it works it is no problem, if the vacuum is unable to break for whatever reason then there is a possibility of contaminants. Then again, if you do not wash your hands well after handling anything to do with the black water then you risk contaminating the potable water supply just as easily, if not even easier.
I personally have never stayed at a CG with "Full" hookups so I have never had the choice. I feel if I did I would still stop at the dump and proceed to use the flusher there tho instead of at the camp site.