I will just add that this also works for Ubuntu ARM 9.04. The 9.04 version is important. I chroot into Ubuntu from time to time in order to compile code or use the full array of Linux tools beyond the few included on the Kindle. I didn't try to get X running at all, though. But some have been somewhat successful:
http://hackaday.com/2009/09/03/ubuntu-9-04-on-kindle-2/