OK people, just because you're sweating, does NOT mean you are burning calories. Just because you're not sweating doesn't mean you're not burning calories. Caloric burn depends on the intensity and duration of the exercise.
The environment plays a big role in how much you sweat. If you exercise in an air-conditioned room, you will not sweat so much because the cold dry air works to quickly evaporate your sweat. Your body can easily cool the heat generated by your exercise.
Exercising in a hot and humid room will just exhaust you faster, and you won't burn as many calories because you will stop working out sooner.
On the same note, when you go into a sauna, you're not burning calories - you are merely sweating. Any weight loss incurred is likely to be water-loss from perspiration, and will be regained as soon as you drink fluids.
Sweating is simply a way to cool your body so you don't overheat. Some people prefer to workout in ways that make them sweat, because they feel like they've worked hard. Other folks like to sweat because it's cleansing, and releases toxins from their body.
So now we know, a good workout and a good sweat do NOT go hand in hand.