Current RepRapPro Huxley and Mendels use a fibreglass-reinforced black rubber MXL belt, 1/4" (6.35mm) wide, on a printed 17-tooth pulley, giving 92.635 steps per mm.
Older machines, pre March 2013, use a steel wire-reinforced polyurethane T2.5 belt, 5mm wide, on a printed 14-tooth pulley, giving 91.429 steps per mm.
We changed over to the MXL belt because it is quieter, lighter, just as strong, and cheaper!
Ian
RepRapPro tech support