Hello,
We now have the Arduino Uno in stock. It appears that the Uno uses a rail-to-rail op-amp, so the threshold voltage of the MOSFET should not matter any more. Since the schematic still shows the same non rail-to-rail op-amp as on the Duemilanove, we do not know whether it is being used specifically to address this issue, or even if it is present on all Uno boards. The unit I tested seems to work as designed, but we are still not happy that the design sends current back into the USB +5V line below about 7.4V.
-Paul