Raspberry Pi and Arduino linked via I2C | Quick2Wire
It’s not quite as bad as I thought. Apologies, I misread the Atmel spec (8161D–AVR–10/09). For a 5V (e.g. Arduino) system, the required input High voltage is NOT 0.7Vcc but 0.6Vcc (i.e. 3.0V) so the 3.3V I2C bus has 0.3V noise margin if everything is good...