On the serial page of the reference
http://arduino.cc/en/Reference/Serialit states
Pins 19 and 18 are also connected to the corresponding pins of the ATmega16U2 USB-to-TTL Serial chip
I don't think this is the case. Pins 18/19 are Serial 1, Serial 0 is connected to the USB chip and that's on pins 0 and 1 as per normal.
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Rob