Purportedly, the chemical compounds within the plastic will begin to "seep" into the water... this is also why they say "don't microwave or freeze waterbottles," as it supposedly frees up substances present in the plastic contacting the water and releases them into the water itself. This seems to be an issue with water bottles containing BPAs, which I do not believe have yet been directly tied to ailments in humans... but have been observed to cause some animals to change gender and other fun things.
The water bottle conspiracy continues...