What code are you throwing (number)? Before I would pay those jerks at the dealer, I would try cleaning your IACV (Idle Air Control Valve). This is a very common problem on our and all cars, where carbon builds up on the spring and causes it to stick. It's very simple to do, and heres a link w/ pics.
http://www.nissanforums.com/showpost...99&postcount=8 . I would give it a good cleaning, clear the codes, and drive it around and see if it throws the code again. Either way, I don't think it is the ECU, and would take it to somewhere else if the code comes back. And cleaning the carbon off the IACV and Throttle Body is something that should be done annually.
Oh and you can adjust the idle, there is a screw on the IACV under a black cap, just unplug the TPS (Throttle Position Sensor) and then screw it I believe clockwise to lower the idle, then plug the TPS back in.