Yep they are very bad for dogs - don't know how many they have to eat though - DD read it in a mag at the vets.
Says this on a website - I think you should ring the vets!
Grape and raisin poisonings in Dogs:
Recently, there was a letter in the AVMA Journal from Dr. Gwaltney-Brant and others at the ASPCA Animal Poison Control Center discussing grape and raisin poisoning in dogs. Apparently, grapes and raisins can be toxic to dogs when ingested in large quantities.
The grapes and raisins came from varied sources, including being eaten off the vine directly. The dogs exhibited gastrointestinal signs including vomiting and diarrhea and then signs of kidney failure with an onset of severe kidney signs starting about 24 hours after ingestion of the grapes or raisins. The amount of grapes eaten varied between 9oz. and 2 lbs., which worked out to be between 0.41 and 1.1 oz/kg of body weight. Two dogs died directly from the toxicity, three were euthanized due to poor response to treatment and five dogs lived. Due to the severity of the signs and the potential for death, the veterinarians at the poison control center advocate aggressive treatment for any dogs suggested of ingesting excessive amounts of grapes or raisins, including inducing vomiting, stomach lavage (stomach pumping) and administration of activated charcoal, followed by intravenous fluid therapy for at least 48 hours or as indicated based on the results of blood tests for kidney damage.
I have fed my dogs a few grapes every now and then for years, so I don't think there is a need to panic if a dog eats three or four grapes but if the whole bunch is missing from the table one day, it would be good to think about watching for any signs of a toxic reaction.