While it's up for debate just HOW lethal these substances are to dogs, experts agree that some dogs are especially sensitive to the compounds while others seem to be able to take in larger amounts and suffer little or no negative effects, mostly depending on the weight of the dog and the type of chocolate. Baker's chocolate is very high in theobromine, while milk chocolate, which is more commonly found in holiday favorites like chocolate bunnies is less dangerous. When I was younger, my family dog, Bootsy, got into an Easter basket and ate the better part of a chocolate bunny, seemingly without suffering any ill effects. She wasn't especially large and was a mixed breed, so toxicity may depend on other factors as well.
Symptoms of chocolate poisoning include vomiting, diarrhea or hyperactivity which can come on very quickly or take several hours to appear. In severe cases, this can be followed by This can lead to hyperthermia, muscle tremors, seizures, coma and even death.
If you suspect chocolate poisoning, it's important to administer first aid to you pet as soon as possible. Steps include inducing vomiting and feeding them an activated charcoal slurry, which absorbs the toxins. More detailed instructions can be found here.
And here are some additional resources:
Happy Easter, Passover, and belated Vernal Equinox!