Has anyone seen the TV reports on this? The story started in Houston and is spreading all over the country. The Marble Institute of America has issued several warnings to it's members to not answer any media questions.

KHOU Dangerous Granite Countertops

I would not be surprised that some tops have gamma rays emitting from them. I had a house several years ago that had elevated radon levels. After we had the house fixed with sub-slab ventilation the numbers were still there. It turned out that the radon was coming from the stone fireplace! The stone had to be removed once done radon was gone.
Granite may contain minor particles of material that gives off gamma rays. You can purchase a low cost radon test at your local big box supply store for around $25. Simply set the test kit on the counter and leave it there for a few days. Send it in and you should find out of you have an issue.

When you read the article, you find out that it's only a very small minority of the samples tested that gave off radiation or radon, and then many at very low quantities, so don't panic. Simple testing will reveal whether your particular stone is emitting anything harmful.