Paul__B:
Not sure what you are describing here. The Nano is essentially a miniature version of the Duemilanove. It is true that if the FT232 chip was damaged it would not download sketches, but when you say "When I unplug the Nano board the USB is working fine", exactly what do you mean?
I have build an board with sensors and power where I plug the Nano. When the Nano is plug and powered by this board the FTDI chip does not work. But when I unplug the Nano board and connect only the USB cable on it, all is working.