A study in the in the Journal Of Human Kinetics showed that stretching before a weight-lifting session will negatively affect your performance in the workout. The interesting part of this news is that the researchers examined both static stretching as well as ballistic stretching (bouncing into an extended range of motion).
Both types made the lifters weaker during their training than when they used light weightlifting to warm up. However, many strength coaches believe that stretching your muscles away from your training (on non-training days, for instance) can help your long-term strength development. As Iron Man has reported previously, stretching that antagonist to those being trained can actually help boost the strength of the muscles being worked.