From the Too Much Information department...
Mofaha is right that those white dog faeces have been gone for quite a while -- since the late 80's in fact. Back in the day dog food was made mostly of bone meal that remained after beef was processed at the factory. The bones were ground up into a fine powder, which was mixed with rendered fat, protein, and vitamins, then compressed into happy little dog food pieces. Dogs ate the food, and crapped out relatively normal turds composed mainly of undigestable bone meal. Bilirubin, urobilinogen, and stercobilin are what give faeces its coloration, and they decay in nature much faster than bone meal does. So, when those colorants leached away, all that was left behind was the white calcium "skeleton" of the poo.
At any rate, in the late 80's the cosmetic industry started using up all of the fat content that had gone into dog food in the past, forcing dog food manufacturers to move to vegetable oil as a binding agent. Vegetable oil didn't stick to the bone meal quite as well, so corn meal started to be used for part of the content. Also, good ol' Ronald Reagan passed a bill that allowed hot dogs' bone content to go up from "trace amounts" to a whopping ten percent by weight. This immediately depleted the bone meal supply available to dog food manufacturers, who switched over to solely corn meal filler.
In a sense, the white dog droppings of days gone by are now in your lunchtime frankfurter.