Commuting as a workout depends on a few factors. One is the intensity. Sometimes I push myself when I ride. Green lights are opportunities to accelerate hard and hills mean I just have to pedal harder. If I feel it in my legs when I'm done I know I had a good workout. But sometimes commuting is just to get me there... I"ll coast down the hills or use easier gears when climbing.

Another factor is the duration. When my commute was about 25 minutes I didn't lose an ounce. I actually gained... and it wasn't all muscle. But last summer I moved and my commute time doubled. Since then I've lost 10lbs in 6 months. I'm sure if I kept a better check on my diet it would have been more.