I had a freak-out moment about this a month or so ago (my husband was eating some grapes and I found out he didn't know grapes were harmful to dogs -- how could he NOT know that?!!!). I'm guessing one or two won't be a problem, but scarily enough, I've read accounts of as few as seven grapes causing harm.
Were they seeded or seedless? I don't know if that makes a difference (probably not, since raisins are also poisonous). There's also speculation about the cumulative effect... e.g. perhaps the dog who got sick from eating 7 grapes had eaten grapes before and perhaps was affected more severely because of that.
"Grapes or Raisins Although the minimum lethal dosage is not known, grapes and raisins can be toxic to dogs when ingested in large quantities. The symptoms are 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. The amount of grapes eaten varied between 9 oz. and 2 lbs., which worked out to be between 0.41 and 1.1 oz/kg of body weight. It has been reported that 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 National Animal Poison Control Center (NAPCC) advocate aggressive treatment for any dog believed to have ingested excessive amounts of grapes or raisins, including inducing vomiting, 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."
Nobody knows what it is about grapes, there have been no studies about it. Some dogs died of kindey failure after eating huge amounts, and some died after eating only a few. If there is a toxin in the grape itself, it's never been identified. Most dogs can eat grapes with no ill effects at all, which further confuses the issue. Is it some kind of allergy some dogs have, but not others? Who knows.
To be safe, I wouldn't feed them to dogs until someone gets around to actually figuring out what the issue is. Personally, I find the complete lack of data or actual research about it pretty frustrating.