I've written a tutorial on the communication between Pd and Arduino, aimed at Pd users who want to program Arduino without using the Firmata firmware.
You can get it here http://drymonitis.me/code/ (look for the specific link).
The .zip file contains the tutorial .pdf, all Arduino sketches, Pd patches, as well as Fritzing files of the circuits.
The tutorial is split in ten projects which are the following:
1 - digital output
2 - digital input
3 - analog input
4 - analog output
5 - analog and digital output
6 - more than one analog inputs
7 - servo motors
8 - analog and digital input
9 - multiplexing
10a - output shift registers
10b - input shift registers

Feedback is welcome : )