There are several ways to connect an Arduino to a Pi. The simplest way is to just plug it into one of the USB sockets on the Pi. Peter Mount, however, needed to do things differently as he was already using his Model A’s USB port for something else. So, he went with the I2C bus. He’s detailed the process over on his blog, including the code you run on the Arduino and on the Pi.
You’re a lifesaver. I just spent about an hour looking for good ways to do something similar, gave up and decided to cheer myself up by reading Pi blogs. And lo and behold – just what I needed! Thanks for the link.