I2C on arduino is tough since I could find no tutorial.
Look for TWI (two wire interface). I2C is a proprietary name, TWI is what Atmel calls it. Arduino provides the Wire library (#include <Wire.h>)
There are examples on the wiring.org site (their library is called TwoWire, but it's the same as Arduino's Wire).
Just today I got a toy example working: a slave Arduino generating a sequence of characters, with a master Arduino reading them via I2C and printing them on the serial port. Not terribly useful, but a good first step. It is doable.
-j