As more and more people get a Pi they are asking how to interface it to their robot. I do not own a Pi but I looked at the GPIO pins available for interfacing. Apart from general digital I/O pins you have I2C, SPI and Serial interfacing available. I assume there is a library or something that allows these pins to be easily access from within the Linux operating system.
So the question becomes do you just use another MCU such as an Arduino to provide the necessary I/O functionality or do you use a custom shield?