That's a good point. Is the switchover from headset to speaker in software (like it is on my Thunderbolt phone) or in hardware?
I know on my Thunderbolt if I plug in a headset with a mic I get an icon on the top of the screen, and if I plug in a basic headset I get a different icon.  There's also a brief delay until the sound switches over, I'm guessing as the sensors determine what is hooked up.  Also, when I unplug the headphones, any music I have playing pauses automatically.
However, on the Nexus 7, there's no sign of any icons when I plug in a headset.  I've honestly never tried a headset with a mic.
I'm wondering if the Nexus 7 might just have a cheap hardware-switched headset jack instead of a software-controlled one to save build cost?