most toothed belts are 5 ply with incredibly strong "strands" running the length of the belt. They can handle enormous torque loads, far more than any cyclist will put out.
one benefit of belt drives is that they are silent and there is no bouncy chain syndrome when riding over big bumps etc.
Some guy in Vancouver has a stretch cruiser lowrider about 12" long, with a 6' long belt drive. Looks pretty damn cool with the billet aluminum toothed pulleys.