I can't post anything with links.
I guess the moderators are on vacation...

Anyway, if you have the skills and equipment to build your own you can get the schematics, board layout and code from my blog (the link is in the engadget article).

With so many people telling me they want one I will have to put some more effort into making the controller prettier and easier to manufacture (without so much hot glue :P).

As for the firmware, most of it is from the V-USB project, I only wrote a little bit of code to handle the buttons and joystick input.
It emulates a standard USB keyboard, so all you need to run it is kernel-power and USB host utils.