There has been much buzz about the chemicals in plastic water bottles. I've heard that freezing a disposable water bottle can be bad for you.
If you're worried about chemicals like dioxins seeping out of the plastic and corrupting your water, have NO fear. This is just an urban legend, a bad rumor, and not at all true.
Actually, freezing your water bottle will work against the releasing of chemicals, which do not diffuse very well in cold temperatures, like in your freezer.
Another fact to set your mind at ease: Most plastic water bottles, and microwave containers do NOT contain the chemicals that form dioxins. Also, in order to form dioxins, those chemicals need to be at extremely high temperatures (above 700 degrees).
Still not convinced? The FDA said "With regard to dioxins, we have seen no evidence that plastic containers or films contain dioxins and know of no reason why they would.”
So freeze and microwave your little heart out. The only thing I'd worry about is using that disposable water bottle over and over again - there's no way to clean it or get rid of bacteria.