In 2004 a vet in Danville, Ohio was treating a 56 pound 5 year old Lab that had eaten a half of canister of raisins and began vomiting, diarrhea, and shaking. The vet had heard about grapes and raisins being the cause of renal failure.
Upon checking, her suspicions were confirmed by MedVet and the ASPCA and she began the treatment recommended by these two agencies. In spite of the recommended treatments and anti-vomiting medications he was euthanized two days later.
The ASPCA Poison Control advises that grapes and raisins appear to cause renal failure in dogs after ingesting larger amounts. It is still unknown if ingestion of grapes or raisins over a period of time will have the same affect as ingesting a large amount at one time, so it is advisable to avoid giving your dog any grapes or raisins period.
Visit the ASPCA site at http://www.aspca.org/site/PageServer and click "Animal Poison Control" in the left hand column for more info. They have several lists of "people food", household plants and household items that can be toxic to pets.
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