CrossRoads:
Thinking a little more - are you considering 2 MOSFETs with supply-load-Drain-source-drain-source-Gnd?
Maybe if you PWM'd both, one from a 555 putting out pulses, and the other from the Arduino.
It is high power. 2Amps. The point of this is I'd be able to give a finished lighting product to someone with a preset time and dim sketch running. So I don't want the end user getting into the sketch because the LED is to bright when its at full power. So I would like to wire in a pot (not sure if it would use a mosfet or not, I guess thats the advice I'm looking for is how to wire this in) so they can turn down the overall brightness but my code would still be intact and thinking its running at full power.