I’d contest the statement that “less than the normal background radiation you get from rocks and stuff”. Go find a Geiger-Mueller counter and aim it at a red Fiesta plate. We did a whole undergraduate lab on exactly that-- figuring out (quantitatively, by counting statistics) whether a source was more radioactive than the background. Our orange Fiesta plate was a significantly higher source (of course, that in itself doesn’t mean it’s dangerous to be around the plates-- we didn’t compare our readings with any safety standards).
The company officials may dispute that it’s dangerous to eat off the plates, even acidic foods, but they have a responsibility to their company. I’d be interested in hearing about an independent investigation.
The reason I looked up the subject on Straight Dope is that not too long after that physics lab, a couple of us geeks ordered a pizza at Dave & Buster’s, and the pie arrived with a stack of Fiesta plates, one of which was pink. (Quick, to the Batcave!) We snickered about warning the waitress, but it would be nice to know how well that old glaze holds up under restaurant-strength usage.