Not just harmful, can be fatal.  It is sort of inconsistent though as it
seems that cats (or dogs) can eat it on and off on occasion but onetime it
will seem to all kick in at once and kill the animal.  Definite NO NO for
any cat or dog.
Larry Osborne

http://www.talktothevet.com/ARTICLES/DOGS/chocolatetoxic.HTM

We've all heard it, "Don't give your dog chocolate it will kill him". We'll
how true is it you're probably wondering. Do I have to rush him to an
emergency vet if he ate one of my M&M's?

The truth is chocolate contains theobromine that is toxic to dogs in
sufficient quantities. This is a xanthine compound in the same family of
caffeine, and theophylline.

Toxic Levels

The good news is that it takes, on average, a fairly large amount of
theobromine 100-150 mg/kg to cause a toxic reaction. Although there are
variables to consider like the individual sensitivity, animal size and
chocolate concentration.

On average,
Milk chocolate contains 44 mg of theobromine per oz.
Semisweet chocolate contains 150mg/oz.
Baker's chocolate 390mg/oz.

Using a dose of 100 mg/kg as the toxic dose it comes out roughly as:
1 ounce per 1 pound of body weight for Milk chocolate
1 ounce per 3 pounds of body weight for Semisweet chocolate
1 ounce per 9 pounds of body weight for Baker's chocolate.

So, for example, 2 oz. of Baker's chocolate can cause great risk to an 15
lb. dog. Yet, 2 oz. of Milk chocolate usually will only cause digestive
problems.

Clinical Signs

Xanthines affect the nervous system, cardiovascular system and peripheral
nerves. It has a diuretic effect as well. Clinical signs:

Hyper excitability
Hyper irritability
Increased heart rate
Restlessness
Increased urination
Muscle tremors
Vomiting
Diarrhea

Treatment

There is no specific antidote for this poisoning. And the half life of the
toxin is 17.5 hours in dogs. Induce vomiting in the first 1-2 hours if the
quantity is unknown. Administering activated charcoal may inhibit absorption
of the toxin. An anticonvulsant might be indicated if neurological signs are
present and needs to be controlled. Oxygen therapy, intravenous medications,
and fluids might be needed to protect the heart.

Milk chocolate will often cause diarrhea 12-24 hours after ingestion. This
should be treated symptomatically (fluids, etc..) to prevent dehydration.

If you suspect your pet has ingested chocolate contact your Vet immediately!
They can help you determine the the proper treatment for your pet.

LOL!  Why do you think there is a cake named, "Death By Chocolate"?  <g>