The question has been posed: When is it best to do cardio, before or after weight training?
Really there is no sure fit answer to the question. It really depends on your body, the results you are looking to get and how long (and hard) you're doing the cardio.
Before: It is best to do your cardio workout before weight training if you're training for something endurance related (such as a 5k). Putting cardio first allows your body to give the most energy and effort into the session so that it can produce the best results. However, if you're going to do a low intensity cardio workout then it can serve to warm up the muscles. Then you can move right into high-intensity weight lifting without having to do as many warm-up sets.
After: It is probably best to do your cardio after weight training if you're looking to shape your body (for bikini season, perhaps). The reason is that a high intensity cardio workout could actually take away from an effective weight-training workout. You need carbs for effective weight training and cardio burns both fat and carbs. So if you do cardio first, you may not have the carbs you need to have a beneficial weight training session. Furthermore, increasing your lean mass will help burn fat because lean mass is what will help use up calories all day long. So if you are looking to build a little muscle, and get killer arms like Cameron Diaz, then weight training should take priority in your workout.