Today I've done some hacking on my Kindle 2. I have done this and that nothing really cool yet. The first thing I needed to do was to re-enable USB networking since this is disabled in 2.3. The link below enables SSH and telnet on the 2.3 firmware. The second link is a presentation from OSCON, the interesting things are the infos about the framebuffer and the keyboard. Have fun!
Re-enable usbNetwork on Kindle 2 firmware 2.3 usbnetwork23-0.10.tar.gz
Hacking your Kindle a talk from OSCON.