This is an ARM version of Ubuntu 12.04 (LTS) patched with the lastest NVIDIA Tegra Drivers.
To install, copy the MROM file (https://github.com/gabrielrcouto/nexus7-ubuntu-12-04/releases/download/v1.0/ubuntu-12-04.mrom) to you tablet and use MultiROM (http://forum.xda-developers.com/showthread.php?t=2011403).
You'll need an OTG cable to plug a keyboard on your tablet and setup it.
Username: ubuntu
Password: ubuntu
$ wpa_passphrase the_name_of_your_wifi_network the_passphrase > wpa.conf
$ wpa_supplicant -B -Dwext -iwlan0 -cwpa.conf
$ sudo su
Password: ubuntu
$ apt-get update
$ apt-get install ubuntu-desktop gnome-session-fallback
After finishing, reboot and you'll see the LightDM screen.
I recommend you to use the "GNOME Classic (No effects)" session, it's light and doesn't have the touch bug, reported on https://bugs.launchpad.net/ubuntu-nexus7/+bug/1068994
If you want to patch your own Linux Distribution with NVIDIA Tegra drivers, look here: https://developer.nvidia.com/linux-tegra-rel-16. The Nexus 7 (2012) uses Cardhu drivers package.
Gabriel Couto - @gabrielrcouto