I wouldn't heat up things in plastics, but remember that polypropylene (number 5 on the bottom of the plastic) and high density or low density polyethylene (number 2 on the bottom) have been shown to be safer and not leech as many estrogen mimetics. There was some issue about polycarbonates (lexan bottles that are so popular) leeching bisphenol A, but most believe the dose is too low to be of any significance. The way I see it, if people are active enough to require a Nalgene bottle, the activity and lack of sedentary lifestyle "probably" more than compensates for the small amount of estrogens.

Eric