Here's the first test release of Pduino, the Pd firmware for Arduino and a matching Pd object, [arduino]. This allows you get data from the digital and analog inputs, and send data to the PWM and digital outputs. It also allows you to change the digital pins from input, output, or PWM. And you can choose how many analog inputs and turn on/off digital input.http://at.or.at/hans/pd/objects.html
Keep in mind its a test release, the only thing I am not sure about right now is digitalInput, everything else seems to work fine. In the future, I would like to add the pulseIn functionality also.
Plus, last but not least, this firmware is by no means tied to Pd. It could easily be used with Max/MSP, Processing, Java, whatever. I'd like to join other people in working on this firmware for any platform. Then the Arduino can be a plug-n-play sensor board in addition to all the rest.