Marc,
the link below is very informative about naked shorting.
When a stock is shorted those that short the stock
have to pay the divdend, when it is issued. Now if
more divdends per share are collected than exist in the known float the differnce is the number of naked short shares. Since the number of legal share holders are known
then there should only be so many shares available for shorting.
http://www.ncans.net/shortemail.htm