With a few precautions there is little to fear!
There was a e-mail hoax sent out some years ago that stated Johns Hopkins has sent information concerning Dioxin Carcinogens causing cancer... Especially breast cancer. The report said Dr. Edward Fujimoto from Castle hospital was on a TV program explaining this health hazard.
Both Johns Hopkins and Castle Hospital have reported the e-mail as false.
Here are a few helpful sites to provide you with the latest information concerning the safety of plastics and their usage in the microwave.