You've got to learn to not believe all the nonsense you see on the 'Net. An excerpt from http://www.hoax-slayer.com/plastic-cancer-link-hoax.html follows:
An earlier variant of this hoax referred only to the freezing of plastic water bottles. This later version has added on spurious information about using plastics in microwave ovens as well. Both versions contain false and misleading information.
Rolf Halden of the Johns Hopkins Bloomberg School of Public Health has expertly debunked the rumour linking plastic bottles to cancer. According to Halden the claim is an urban legend. He explains that:
Freezing actually works against the release of chemicals. Chemicals do not diffuse as readily in cold temperatures, which would limit chemical release if there were dioxins in plastic, and we don't think there are.
Experts also contradict the claim that using plastics in microwaves can cause dioxins to leech into the food. According to Edward Machuga, Ph.D, of the US Food and Drug Administration (FDA), "The FDA has seen no evidence that plastic containers or films contain dioxins and knows of no reason why they would". The general consensus is that using plastic containers or plastic wrap in microwaves is not dangerous, so long as microwave safe plastics are used and manufacturers guidelines are followed. The FDA article does admit that substances in plastics can leach into food. However, the FDA does not consider this to be a significant risk to humans.