Here is a sample of what you see on the serial port during boot:
http://igorsk.blogspot.com/2007/12/kindle-boot-log.html
You can see a lot of that same information by typing a 'dmesg' command at a root prompt in the kindle (either USBnet telnet or ssh prompt, or Luigi's kindle terminal -- I prefer myts-6)...
The special thing about serial port access is that certain key presses sent over the serial port at the right time let you get into a diagnostic menu, where all the *real* magic happens.