I have come across these a couple of times and only now discovered what they are ..... a Pill Millipede (Sphaerotheriida). They curl up into a ball like this at the slightest hint of danger so naturally one cannot see that they have two pairs of legs per segment which would identify them as millipedes. Next time I find one I will take the time to be more careful and wait until it unrolls itself so I can get a better picture. :) It is probably about 3cm (1 1/2") in length.
