First, let's look at the connector itself, so you can figure out which end is up. With the rear cover removed, you can see the keyboard connector in the lower-right corner of the main board. It's a thin white FFC (Flat Flex Cable) type, and has 20 very small pins.
As seen in this picture, pin 1 is at the bottom, and pin 20 on top.
The keyboard itself is nothing more than a bunch of dry-contact switches, which is to say there aren't any sensitive electronics to worry about damaging or cloning. To simulate a keystroke, all you have to do is short two wires together. The trick is figuring out which pairs do what...
For your consideration, I present the keyboard map of the Kindle-3.
(Format: (Short this pin) (to this pin) (to generate this keystroke) )
A simple pushbutton can be used to mimic the stock Kindle keys.
One word of caution - the auto-repeat delay is very short. You don't notice this on the stock Kindle because those tiny little buttons don't maintain contact when pressed. I won't pretend to know much about their internal structure, but I can tell you that a standard button will generate a lot of duplicate keystrokes in rapid succession if held down or even pressed slowly. In my case I'm using a microcontroller to handle all the timing so it's not a big issue. However, if you're just cloning the keyboard with no additional electronics, you might want to investigate a debounce circuit of some sort or you'll get a lot of duplicate keystrokes.
That's it! Once again, thanks for reading!