Smudge has got it right - all to do with bones. In my childhood most dog owners would buy bones from a butcher and dogs would spend many a happy hour sucking off (sorry) the last remnants of raw meat and gradually demolishing the entire bone. These days dogs seem to get fed out of packets and tins, and rarely get their teeth round a good bone... Less bone, less white poo.
No I seen some too! I stopped and stared at it for a bit I was temped it take a picture and print it off just incase I fancied a chance from looking at the normal brown colours of the clutches of dog eggs my two leave behind I could simply just place a printed picture over them for a much more pleasing view to the eye.
Perhaps we can have a look I've found a white dog shite thread and post some pictures