Polypropylene does seem reasonably safe right now... but it is known to leach some chemicals, and don't forget that polycarbonate was touted as super-safe by the "natural living" crowd until maybe five years ago. I'd stick with glass, stainless steel, or (lead-free, obviously) ceramic.
I just have to throw in the Endocrine Society's position statement on endocrine disruptors, then I'm done.