[geekmaster]

@Sir Alex: The KDT tool does not work for me. I have multiple kindles and they each have a different IP address (set in the usbnet/etc/config files). Perhaps KDT *assumes* that all kindles use the default 192.168.2.2 address. Unfortunately, I see nowhere in KDT that lets me change this address to what my kindle is using.

@akleiw: If you can telnet or ssh into the kindle (from any Operating System), you can simply do:
cat /proc/cpuinfo
to get the serial number.

EDIT: I just discovered that this only works on the Kindle 3. Version 2 firmware does not display the serial number in the "cpuinfo". Also, the root password on version 2 firmware appears to require 4 characters of the md5sum instead of 3 characters as needed by version 3 firmware.

You can use that to get the root (recovery) password:
echo fiona$(grep eri /proc/cpuinfo|cut -c12-27|md5sum|cut -c8-10)