Chris,
Thanks for encouraging me to persist.. Due is now acknowledging the addresses of my I2C devices.
Question:
I am using a DS1307 RTC module on my I2C bus. It requires Vcc of 5V. I am feeding its I2C lines at 3.3V with 2.2K pullups. The DS1307 wants to drive its own I2C pins at 5V, but I am assuming the pullups keep the Due from seeing the excess voltage. It appears to be working smoothly. But in the long term, will this cause trouble for the Due or the DS1307? I could insert a logic converter.