Jaychad:
Thank you thats very informative. How should these resistors be attached?
Very carefully.
The standard "Wire" library turns on the Arduino's internal pullup resistors. This is wrong, it will apply 5V to your 3.3V devices.
Worse, if you add external 3.3V pullups then you have the 5V and 3.3V pullups on the same lines.
So the first thing you need to do is:
a) Modify the function "twi_init()" in file "twi.c" to fix this.
or, second best:
b) Do "digitalWrite(SDA,0)" and "digitalWrite(SCL,0)" right after you call Wire.begin()
After that, just join all the SCL and SDA pins of your devices to A4 and A5 on the Arduino then add a 4.7k resistor from 3.3V to each of the lines (one resistor per line).