Make Blog, but am not sure if this guy is a genius of a nutter.
He has hacked his kindle into a USB connected display for his PI, which is an interesting technical challenge, but can easily turn e-book reader into scrap.
Kindles run a variation of Linux, so in theory hacking them is quite possible, however there is no "reset to factory" options, so if you lock it up you are royally stuffed.
This one is not for me, I love my kindle and would not risk it for anything so if I wanted a small, portable PI I would buy one of the displays I was talking about last week. Its cheaper, doesn't tie up a USB port, but most of all it won't wreck you kindle.
Full details at http://www.ponnuki.net/2012/09/kindleberry-pi/