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It should only take a minute to test. Apple's site is foggy on the details.
 
Can anyone who received their rMBP confirm if this works?

http://support.apple.com/kb/HT3625

Well, I went to the Sound Menu - Input. I plugged in my iPhone headphones/microphone and it automatically switched from Internal Mic to External Mic. Did a quick check, and the sound was being taken from the headphone mic, not the internal mic. So if this is what you were looking for then yes, it works.

Running rMBP - 2.3/16/256 under OS X 10.8.
 
I'm looking to see if it supports "line in" in stereo. Have yet to find a solid report on this.
 
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