I just built a data sheet for the InputShield...
It's only four pages, but it should have all the information needed to use the joy stick and buttons. This will definitely act as a good cheat sheet if anyone wants to replicate Matt's Open Source Game boy project.
The InputShield has 2 modes to control the output and input pins. This feature allows the user to control 2 InputShields from the same Arduino I/O board.
If you have any questions, shoot me an e-mail.