Yeah, logic tells me that any additional electric device (lightbulbs, radio, even windshield wipers) add stress on the alternator.
The AC uses a lot of electricity, that's why most people tend to turn it off when driving fuel efficiently.
But if your radio has 2x50W front speakers, and additional 2x100W trunk woofers, the total power consumed by your radio would be well over 300W. When used at max volume, the radio is coming dangerously close to the power required of an AC system.
Yea what efritsch says, ACs main cause for reduced MPG is not electrical load but the mechanical load the compressor puts on the engine. Although of course the fan keeping the condesor/radiator cool does add a little to fuel consumption.
At highway speeds (not idling), your use of the Alternator's output power is going to have very little effect on your MPG. Might affect it as much as 1 MPG, if you are running a lot of "electrics". This includes things like the fan for your interior, a high powered radio, etc. Electric window defrosters (like on most car rear windows) things like this.
The clutch for your A/C compressor is basically an electro-magnet that pulls the thing together so that the engine can now turn the A/C pump. The pump itself is mechanically driven, not electrical as someone above pointed out, and that in itself is a heck of a load, as the engine has to work harder to turn it to compress your refrigerant and make it cold. Yes it also electrically runs the front fan (depending on conditions and vehicle) more often than it would normally run.
At idle however, the engine is turning a lower RPM. This means the alternator output is also lower, so when you throw a heavier load on the electrical, the engine has to pick up to maintain the output. As someone else pointed out, even rolling down a window can make this happen, but doing that while driving will not as the RPM is higher. Anyways, as an answer to the original question, I would have to say that it won't affect your MPG as much as you would think it would.