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.