I found this at

http://www.landfield.com/faqs/dogs-faq/medical-info/part2/

which seems to have some acurate medical info.


It is not chocolate itself that is poisonous to dogs, it is the
   theobromine, a naturally occuring compound found in chocolate.
   Theobromine causes different reactions to different dogs: dogs with
   health problems, especially epilepsy, are more affected by theobromine
   than healthy dogs. Theobromine can trigger epileptic seizures in dogs
   prone to or at risk of epilepsy. The size of the dog will also be a
   major factor: the smaller the dog, the more affected it is by the same
   amount than a larger dog. Therefore, toxicity is described on a mg/Kg
   basis.
   Furthermore, theobromine can cause cardiac irregularity, especially if
   the dog becomes excited. Cardiac arythmia can precipitate a myocardial
   infarct which can kill the dog.
   Theobromine also irritates the GI tract and in some dogs can cause
   internal bleeding which in some cases kills them a day or so later.
   Theobromine is also present in differing amounts in different kinds of
   chocolate. milk chocolate has 44-66 mg/oz, dark chocolate 450 mg/oz
   and baking/bitter chocolate or cocoa powder varies as much as 150-600
   mg/oz. How much chocolate a dog can survive depends on its weight (and
   other unknown circumstances). Under 200 mg theobromine per kg body
   weight no deaths have been observed.
   Theobromine will stay in the bloodstream between 14 and 20 hours. It
   goes back into the bloodstream through the stomach lining and takes a
   long time for the liver to filter out.
   Within two hours of ingestion, try inducing vomiting unless your dog
   is markedly stimulated, comatose, or has lost the gag reflex. If your
   dog has eaten a considerable amount of chocolate, or displays any of
   the above symptoms, take it to the vet without delay.
   In the absence of major symptoms, administer activated charcoal. The
   unabsorbed theobromine will chemically bond to this and be eliminated
   in the feces. In pinch, burnt (as in thoroughly burnt, crumbling in
   hand) toast will do.

Susan