Can dioxins be released by freezing water in plastic bottles?
There's an email attributed to Johns Hopkins University in the US that contains this statement, plus the one about not using (any) plastics in the microwave. But it is pure urban myth, and the email did not come from the University. The Johns Hopkins University website contains the article 'Researcher Dispels Myth of Dioxins and Plastic Water Bottles', which says: "There are no dioxins in plastics. In addition, 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."
The bottom line: Myth
It's fine to freeze your water in a bottle if you like it nice and cold.