There are pads for an internal SD card slot on the Kindle DX, in the large space under the mini-PCIe slot.
Since there are 32Gb SD cards out there, this could be pretty useful if anyone got it to work.
However, AFTER soldering a surface mount SD card slot to it, and soldering 22 ohm resistors on all the missing resistor pads leading to the SD slot traces (just a guess, they're probably for zero-ohm, but 22 is a little safer just in case)... I finally noticed that most of the IO lines going to the slot are shorted together.
Now, this could simply be that they did not want to leave unused IO lines floating, so there could easily be other zero-ohm resistors somewhere else pulling them to something. The problem is that if this is a common power rail, then it'll be very hard to tell where the pull resistors are.
The pictures I see online of the other side of the board do not show much around that area. I have not tried removing the display from the front of the PCB to check myself, though. (And I don't plan to)
The slot might also be meant to be controlled from one of the unpopulated IC pads.
Anyway, I thought this was interesting and so I wanted to share it. It's not quite worth pursuing further for me, though.
I've since completed my hack (so far) and I've closed it up again. My wife caught me with the brand-new Kindle in many pieces, so now I -really- have to be careful not to fry the thing. I can't play dumb now.
Also, I'd be very interested in if anyone has found pads for an extra USB slot somewhere. ???