Allen Huffman used a Teensy 2.0 as an interface between arcade joysticks and an iPad.
The project came about when he was having difficulty getting an expensive input board to work in a haunted house attraction. When he discovered that the iCade keyboard commands worked over a USB connection, he figured he could use an Arduio board and hood the USB side to an iPad and create his own interface. With a bit of research he got his project up and running.
Be sure to check out his forum post for details on how the project came about. The code for the project is available on GitHub.