Guess I'll jump in and share my experience, which is contrary to the running theme of this thread. I'm a new guy, don't want to piss anybody off, but my experience is my experience.
I gained the "freshman
50" when i went to college in 2000, which brought me to around 235 pounds. For a while I did nothing, but around 2005 I really started trying to lose weight. I tried exercise, in the form of running, walking, and riding a stationary bike. I tried many times and failed to lose any weight. Maybe I was doing it wrong, but I started at 20 minutes 3-4 times a week and eventually would get up to 30 minutes 3-4 times a week, with intensity. It was frustrating to not lose any weight, and put in so much work. Exercise made me feel good, but it wasn't getting easier, and the scale wasn't changing a bit.
Eventually in the spring of '08 I was whining to a friend and mentor of mine about how I was tired of being fat, and he told me exercise was great for overall health, but if I wanted to lose weight all I needed to do was change what I eat. He told me to cut sugar and carbs. So I went extreme and did good old fashioned Atkins. Since then I have slowly converted out of Atkins, and have converted my diet to eating whole, non-processed foods. I avoid fast foods. I eat lots of vegetables. I eat organic shredded wheat cereal in the mornings loaded with fiber. And when I want a country fried steak or some ice cream, I don't torture myself, I just have some! I just don't eat it everyday.
As I lost weight portion size went down. I focus on organic foods, and keep to the outside aisles of the grocery store whenever possible. I avoid foods with ingredients I don't understand (yes, that means high fructose corn syrup and partially hydrogenated soybean oils).
Doing all this lost me 40 pounds, without a single second of exercise (ok, i did some sit ups toward the end of last spring).
Having said all that, I am resolving to start an exercise program of some kind this spring. It may not be the weight loss solution for me, but I do believe its the true "magic pill" for a lot of health problems.
Anywho, just my experience. If you are trying to lose weight and busting your butt exercising to no avail, try concentrating more on the diet.