I think the d3o compound is pretty amazing; I read about compounds that have dynamic properties based on rapid changes in their environment and I found how extremely difficult it is to develop these simple compounds that have "split personalities".

Although it is pretty amazing, I highly doubt that this compound (when in the form of thin pads) would protect a cyclist from a hard fall onto cement. When a foam helmet comes into contact with the concrete, the hard foam disperses the impact across the entire helmet and then when it has absorbed a certain critical amount of energy from the impact it breaks into pieces b/c the foam can't absorb anymore energy without releasing some.
The d3o pads, though hard at impact are very thin and I suspect if I planted my head onto cement the pads would stiffen and absorb/disperse the energy of the fall, but simply not enough to prevent me from getting really hurt. The amount of energy it could absorb/disperse would be too low to prevent your head from being very jostled, if not seriously injured.

If these pads were thicker and could break into many small pieces I could see how they could absorb enough energy to protect your head.