There have been many questions in recent months about the safety of BPA (bisphenol A). Its a chemical used in many polycarbonate plastics in consumer products such as baby bottles, water bottles and food containers. Despite a May FDA report declaring BPA "safe" (after concerns were raised to its safety), now it appears that that might not be true. The National Toxicology Program (NTP) has declared that “some concern” BPA is of “some concern” for effects of development of the prostate gland and brain and for behavioral effects in fetuses, infants and children.
When in doubt its obvious the choice is to stay away from BPA. Here is a great post that lists all the known BPA free bottles and sippy cups from the Family Health & Safety blog.