I've been commuting (about 50 miles per week) for about 3 years. I thought I'd lose weight (I'm packing about 30 extra lbs), but I never did. I finally faced up to the fact that I just eat too damn much food. So, two weeks ago I signed up for Weight Watchers Online (I was familiar with the program from several years ago). I've lost about 5 lbs so far.

So to answer your question (in a way), Weight Watchers allows me 26 "points" per day (food is assigned points based on calories, fat, and fiber). My daily commute (18 miles RT, ~90 minutes) earns me an extra 6 points in food allowance. 6 points is approximately 1 medium slice of pizza, or a turkey sandwich.

In other words, commuting will help you lose weight, as long as you go easy on the cake.