There is another (unprivileged) login on most Kindles: User framework with password mario.
There are different reports with different Kindles and different firmware versions. I have verified the result on a Kindle 4 with 4.0.1 firmware but it should also work on other Kindles and with other firmware versions. Just try the three possible passwords. Good luck!
This script was converted from a little python script to javascript. It uses a BSD-licensed MD5-Javascript library by Paul Johnston.