Hi Michael,
My understanding of the audio port on the MacBook is that one is optical digital audio out/headphone out and the other is optical digital audio in/audio line in. That being said, I use a non-USB headset for Skype that has two jacks, one for listening and one for speaking that occupy both ports when in use. My understanding is that while the iPhone headset supports in & out, the hardware on the MacBook does not.
Matter of fact, I just plugged my iPhone headset into both ports (obviously not at the same time) and got audio from my voice from the input port and audio from my machine from the output port.