The Gameduino board is a Xilinx FPGA - a fancy parallel processing chip - and a few support parts. If you're familiar with Xilinx FPGAs, JTAG and soldering fine-pitch surface-mount components, you should be able to make a Gameduino board. Otherwise, getting started with FPGAs can be a bit of a steep learning curve.
All the design files for Gameduino are open-source.
The Eagle board layout is under http://excamera.com/files/gameduino/eagle/
The Verilog source for ISE Webpack 11.1 is under http://excamera.com/files/gameduino/verilog/
This should be enough to get the Gameduino up and running. To load the SPI flash with the Gameduino image, see Xilinx application note XAPP974.
Here is my first prototype. The PCB was from batchpcb.com.