Gumstix with an e-ink screen
Get yourself a chinese ebook reader running linux.

Go look on ebay. "ebook linux"

I don't know how to root the one commonly sold with a wifi and keyboard yet, but it is definitely sold as a linux machine.

I own an earlier model without keyobard or wifi ( I wanted a book reader.)  It in an ARM CPU, runs linux, takes 8Gb memory cards and internally the PCB has pads for 9 pin serial port, vias for the 9 pin serial port, SIM slot, Ethernet jack ( and pads to attach the Ethernet chips) sound in, input ADC pads next to it, and pads for a microphone. Looks like the development team used their dev layout with less parts loaded. I'd bet the parts used were the cheapest supported by linux/arm.  The screen controller is a standard e-ink interface part too; so in theory I can build my own bootable linux image for it. 

My experience is that the bootloader shows the GRUB line on one line of the display just as it boots.  A serial console connection ( indicited by the presence of the vias for the serial as well as the pads) would allow me to interrupt boot and redirect init=/bin/sh

So then all I need is to mount the removable media ( if it hasn't automounted!) and copy the existing system image off to removable for autopsy and alteration.