I read this interesting article in the NY Times explaining that some granite emits potentially dangerous levels of radiation. It's rare but it's happening more and more as demand for the material rises and suppliers source it from all over the world. They led off with the experience of a woman in NY who hired a Radon Detection expert...
“He went from room to room,” said Dr. Sugarman, a pediatrician. But he stopped in his tracks in the kitchen, which had richly grained cream, brown and burgundy granite countertops. His Geiger counter indicated that the granite was emitting radiation at levels 10 times higher than those he had measured elsewhere in the house.
Radon, a radioactive gas, is a known cause of lung cancer. I don't think everyone should be afraid of thier granite countertops but the article is worth a read, and if you are considering buying one you may want to do a little more research into the source before you drop a few thousand bucks.
The NY Times Article: What's Lurking in Your Countertop