jroorda:
That all depends on where you hook up the ground from the power supply. Say if you plug the power supply ground into pin 4 the current will go from pin 23 to pin 4 and back to the power supply. It will not alter the current flow in the USB cable. This should be fine as the ground traces on the Arduino are quite substantial. If I was driving a large load with lots of current I would make sure large wires connected directly from the power supply to both the positive and negative terminals of the servo and then add a small wire to tie the ground from the power supply to that of the Arduino. It won't carry much current; it just keeps the two supplies from drifting apart.The control line for the servo (13) is just a data line and carries a few milliamps at most.
Sorry for the late reply, but thanks a lot for your input!
One more question though; is it safe to power the Arduino by a 12V RAW input while at the same time use the FTDI cable to communicate with the board? Note that the FTDI also supplies electricity.