I've discovered (the hard way) that the USB pins on the arduino schematic are mapped backwards.
They should be:
2 USB data -
3 USB data +
But unfortunately in power/gnd and the two data lines are reversed, which then reverses the pins on the board layout.
So if you are going to develop something based on the arduino board schematic, then just be sure to disconnect the connector in the schematic, flip it, and then reconnect it.