Digg:

Here in the US, the Food and Drug Administration is responsible for regulating what is microwave-safe. A plastic dish that meets their approval will say “microwave-safe” somewhere on the container, or feature a square icon with wavy lines.

The thing is, the FDA is not going out and testing every single plastic dish on the market, they’re just set the testing standards for a microwave-safe dish. It’s up to the manufacturer to test to the FDA’s standards, and then label their goods as microwave-safe. In other words, there are microwave-safe containers, and then there are containers which have yet to be proven as microwave-safe.

Just to be on the safe side, I never microwave anything in the container it comes in.