I've been following this project for a while, their goal is to produce a small ARM board with everything you need to program it built in for only $5 -- the idea being that at that price, you can stick them everywhere and not have to worry about the cost. They are now close to being finished, and are trying to raise enough money to get some prototypes out into the hands of hackers looking to play with them. You can see details on the project here: http://mchck.org/

Features
open: entirely open source hardware and software
powerful: ARM Cortex-M4 with DSP extensions, 50MHz, 32KB code flash, 8KB RAM, 32KB data flash
easy to use: programmable via USB; USB 2.0 full speed device and host in hardware
massive connectivity: 10 PWM outputs, 12 analog inputs, 29 GPIOs, 6 serial interfaces, 14 touch sense inputs
low power: can run directly from a coin cell battery for months
extensible: add LiPo battery charger, 2.4GHz radio, extra flash memory, all without additional boards
small: only 50mm*19mm / 1.97in*0.75in
cheap: $5 to build at home

They need to sell 50 in order to get bulk pricing on stuff, for $60 (which includes shipping) you get:

5 boards + all the parts needed for them
2 RF communications boards (NRF24L01+)
2 LiPo battery charger ICs
2 180mAh LiPo batteries
2 1MB SPI flash ICs
3 different power regulators (so you can run from a variety of power sources)
2 microUSB jacks (the boards themselves will plug into a USB port directly, the optional jack allows you to instead use a USB cable)

I've already ordered a kit, I'm thus spamming you all in the hopes that others will as well and I can get my boards (I have an unhealthy addiction to small inexpensive microcontroller boards)

link to order here --> https://github.com/mchck/mchck/wiki/Prototypes

Note that this is not a finished product yet -- you'll either need a bit of experience with ARM and microcontroller dev, or be willing to wait to use the boards until they finish all the software and iron out any issues that may arise.