This particular flavor of Linux is only distantly related to Android, but it's popular among hackers and it has a sizable development community. There have been previous projects to port it to the Nook Color, and those projects have had varying degrees of success.
The project is still in the early stages, and while Ubuntu is running on the Nook Tablet, it's not using the hardware directly. Instead it's running in an app in the Nook Tablet's original Android firmware, and getting it running is still a fairly complicated process. In order to actually use Ubuntu, first you have to have a hacked Nook Table, a terminal app installed, and more.
You'll need to start the OS and a VNC server from the Nook Tablet's terminal, and then open a VNC window on the NT. When you maximize the window it will look like you're running Ubuntu.
Unfortunately, it's not much use at present because the extra layers of software running in the background slow it down significantly. A lot of Ubuntu apps like Firefox work, but Ubuntu itself is of little or no practical use thanks to lag.
But the developers are working on it. They're hoping to get Ubuntu running well enough that they can crash the underlying Android and keep using Ubuntu. This trick would give it access to the hardware and let it run faster. If they can get that trick to work then it would also let them do the same for Cyanogen Mod.
via XDA Forums