Testing I2C connections between Raspberry Pi and Arduino

Can a 5V Arduino connect directly to a 3.3V Raspberry Pi via I2C?  Yes. Sort of.  You will need to disable the Arduino internal pullup resistors.  The internal pullup resistors of Raspberry Pi, in theory, should make the connection works without using a level shifter.

Rpi          Arduino
---------    ---------
Gnd          Gnd
SDA (GPIO0)  SDA (A4)
SCL (GPIO1)  SCL (A5)

Code also available on github