It is true that rotors are usually the cause. I've never seen calipers cause a vibration. ABS activation does in the pedal, but thats only in an emergency stop.
A tire balance problem would increase with speed and may go away after a certain speed.
Our cars have sealed hubs/bearings and a vibration would occur all the time, not just when braking.
I've never seen rear brakes cause a vibration, but I suppose it could possibly if they are really warped. What do the rears look like? Are they shiny like a mirror or badly grooved? Also do you have rear drums or rotors?