So in summary, is this correct?
If using Pin 20 and 21:
Internal pullups are provided.
Use Wire.setup() to control these.
If using SCL1 and SDA1:
You must provide 1.5k pullup resistors
The pullup source is taken from the 3.3V pin near the RESET pin.
Use Wire1.setup() to control these.
If this is all true, then I am unable to get pins 20 and 21 working. They just stay at logic high all the time. Have I missed something? I'm just running the master code here http://arduino.cc/en/Tutorial/MasterWriter
Will this work with just one device? I am just using one device and a logic analyzer to monitor the pins.