The Nexus 7 has a normal 3-pole headphone-only jack at the bottom, and two microphones: one on the top left (on a little daughterboard plugged into the mainboard) and one on the lower left edge (soldered directly to the mainboard) about 1" above the docking pins.
I don't know why they chose to do that. It is possible to replace the jack with a 4-pole and reroute the top microphone through it so it behaves as you would expect, but it's not a simple hack and it requires a willingness to shred your warranty and the equipment & skill to work with small SMD parts and flex-cable. Also, physically fitting and securing a 4-pole jack will probably involve epoxy and a dremel in addition to soldering little jumper wires.
The Realtek ALC5642 - the (micro|head)phone driver chip - has Echo Cancellation and Noise Suppression built-in; one potential issue is whether the boost gain on the microphones (which is software selectable when the chip is used in PCs) is fine the way it is set, or if it can be tweaked. I haven't yet looked into how Android deals with this, nor do I know if it is a software or firmware setting - I strongly suspect it is software as that is the only way that makes sense, but putting in a 4-pole jack in the first place makes the same kind of sense and they didn't do that.
I suspect the second microphone strips out ambient noise, such as the noise of the speakers - which it is much closer to - when the top mic is being used... that would be an easy way to prevent feedback, but it wouldn't be necessary when using a headset w/microphone. If it doesn't cause any problems I'll leave it alone so everything works the same without headphones.
I'll be tearing into my N7 next week to experiment with this stuff... I'll post results when I have them.