Doesn't physically turn off the internal microphone. I've found out the hard way: I have a headset with a hardware mic switch. If this switch is in the "off" position when headphones are put into the jack, the mic built into my mac still operates.
This is with a mid-2012 macbook model.
Plus on MacBooks there is a second microphone to gather surrounding noise to improve sound quality. I don't think there is any non intrusive way to stop it from activating either.
That's because the switch probably makes it look to the mac as if there's no microphone at all.
You can, for example, set up an aggregate audio device in the audio input/output config panel to enable all inputs and outputs to work at the same time. So it's not unreasonable to think that the internal mic could be activated even with something plugged into the jack.
Well I was playing around a bit more, and I was able to disable the(or a, not sure how many are in 2010 MBPs) internal mic by taking an old audio input jack (cutting the wires), plugging it in (so it looks like a nub sticking out), going to the audio settings and setting the audio port from "Sound Output" to "Sound Input" and the internal mic picked up no readings. Still trying to figure out how to do that from the command line.