http://www.mkiv.com/manual/manualtt/engine...iption/emd3.JPG
This is from the engine workshop manual for the 2JZ-GE, 2JZ-GTE engine. Half way down the page it talks about the piston depressions to avoid interference so there is no real problem with a cam belt break, other than breaking down and needing a tow to home/garage.
The drive belt normally starts to make a noise and you can see cracks in it before it breaks. Again no damage will be done provided you stop the engine as soon as the warning light comes on, otherwise you will overheat the engine as the water pump will not be operating.