Not all plastic water bottles are the same!
Nalgene water bottles are made from Triton plastic. They are BPA free and come from a company that specialises in laboratory containers. Indeed, the water bottle itself is derived from a container used to store pipettes (that explains the beaker-like design).
I cannot say I understand the science behind the Triton plastic, but I do know that the water bottles are strong. Really strong. I have dropped a Nalgene bottle down a 20m rock face and apart from a few scratches the bottle was fine. Since then I have taken to clipping my Nalgene onto my rucksack with a carabiner, the lid is connected to the bottle with a strap which is perfect to place a carabiner through.