Toby did this as a puppy and the vet told us to nip it in the bud immediately. It is called Coprophagia.

Here is what was in the hand-out we were given at the time:

"Common reasons behind coprophagia are dietary imbalance, deficiency, food intolerance or allergy. If your puppy eats food containing ingredients it is allergic or intolerant to, or more food than it can digest, your puppy may not be getting the nutrition it needs, even though eating seemingly well. If food is not digested or absorbed properly, the resulting stool is largely undigested which can make it tempting.

If your dog has a problem with coprophagia, the first step, after a Vet check, is to eliminate the most common problem ingredients from the dog food, which are over-processed wheat, corn and soy. Improving the diet with high quality digestible foods and reducing rich extras can be enough to make a dramatic difference. Adding a digestive enzyme can help your dog process its food better. Some dog owners have found good success by simply adding Apple Cider Vinegar, Spinach, zucchini, green beans or pineapple to the diet or by using commercial deterrents like "Deter" or "Forbid".

Ideally, take your puppy out on a leash each time he goes out to eliminate. After eliminating, call your puppy to you promptly and do not even give him a chance to sniff the droppings. Of course, it will also help tremendously to keep your yard as clean as possible so stool eating never becomes a habit to begin with."

Here are some links to the top three Coprophagia deterrents.

FORBID
http://www.lcsupply.com/store/catalog/forbid.htm

DIS-TASTE
http://www.drsfostersmith.com/product/prod_Display.cfm?CatalogCustomerNum=&%3Bamp%3Bamp%3Bamp%3Bamp%3Bamp%3Bcatalognum=&pCatId=583&prodId=1284&catId=1068395&siteid=12

DETER
http://www.cjshopping-network.com/Pet-Medications/Deter-Dog-Corprohagia-Treatment-60-Tab-Bottle-.html

We gave Toby Deter and in two days he stopped. We had to follow through with the treatment for 4 weeks and he has never eaten poo since!