This is Graham, my wonderful (most of the time) Jack Russell Terrier.  We brought out the chocolate fountain for our hospital Christmas party and you can see who was most helpful at the post party cleanup.  As you may know, Jacks are very quick and this can get them into trouble.  Since this is a common event around the holidays, I thought that I would use the occasion to discuss chocolate toxicity in dogs.  Graham, by the way, is fine.
The signs of chocolate toxicity depend upon both the size of the dog and the amount (and type) of chocolate ingested. A small amount of milk chocolate may have no effect at all, while an ounce of unsweetened baking chocolate can be fatal to a 10 pound dog.  Clinical signs usually occur within 6 to 12 hours after feasting. Initial signs include increase thirst, vomiting, diarrhea, bloating, and restlessness. Signs progress to hyperactivity, increase urinations, weakness, tremors, and seizures. Other effects include rapid heart rate, heart arrhythmias, rapid respirations, cyanosis, hypertension, hyperthermia, and coma. Because of the high fat content of many chocolate products, pancreatitis is a potential secondary problem 24 to 72 hours after eating. Death is generally due to cardiac arrhythmias or respiratory failure.
White chocolate has very low amounts of theobromine, the offending ingredient in chocolate, so while not an ideal nutrient for dogs, it is not toxic.

Compound                                           Theobromine
(mg/oz)                          Caffeine
(mg/oz)
White chocolate                                  0.25                                                          0.85
Milk chocolate                                     58                                                             6
Dark, sweet chocolate                        130                                                           20
Semi-sweet chocolate chips              138                                                           22
Baker/s (unsweetened) chocolate   393                                                           47
Dry cocoa powder                               737                                                           70
Instant cocoa power                           136                                                           15
Cocoa beans                                         600                                                          NA

Based on ASPCA Animal Poison Control Center (APCC) experience, mild signs occur in animals ingesting 10 mg/lb of theobromine and caffeine, severe signs are seen at 20-25 mg/lb, and seizures occur at 30 mg/lb. Accordingly, less than 1 oz of milk chocolate/lb is potentially lethal to dogs; for baking (unsweetened) chocolate, less than 0.1 oz/lb is potentially lethal.