I have a funny feeling its a HDMI port, but only for certain Nexus 7 devices. The Xperia Play Developer version had the same thing, but the normal one they sell doesn't have one. So it may be a "developers" Nexus 7 with a HDMI port.
Well, from the schematics that were posted today, I have a feeling that is a JTAG port, since it's located right beside J1 (a 24pin connector on the main board) that has serial and JTAG IO pins. It would be good for the engineers not to have to keep opening the unit to flash it vis JTAG, which is a LOT faster than flashing over USB.
It's probably a HDMI connector. But HDMI is usually already supported (on other platforms) by running an MHL output over the micro-USB. This would explain why they didn't bother adding that port to production line. (It's always better to have to buy external adapters, right!?)
Wow, nice work digging up an old thread. (look at that date gap...). The presentation N7s had HDMI ports.
Go check out some of the other threads from this date (the day it was announced) and you'll see this question was asked, debated, fought over, and confirmed several times here on XDA.