If you’re wondering how to get I2C running from your MEGA to Duemilanove Arduino boards here’s the scoop:
MEGA Pin 20 is SDA (Serial Data)
Duemilanove Analog Pin 4 is SDA
MEGA Pin 21 is SCL (Serial Clock)
Duemilanove Analog Pin 5 is SCL
Hook up SCL to SCL and SDA to SDA across the boards, also hook up ground between the two boards. You do NOT need to connect the +5V between boards.
You’ll need SCL and SDA to be pulled up (to +5V with resistors, between 2.2kOhm and 47kOhm), it will run without them but its recommended to make your bus within specification.
Example bus from Instructables.
MEGA Pinout information (search for i2c on the page).