From Lyle McDonald's The Ketogenic Diet:

"A very prevalent misconception about aerobic excercise is the so-called 'fat burning zone' which is supposed to optimize fat loss. It is true that a greater percentage of fat is used during low-intesity excercise (see chapter 18 for details). This suggests that low-intensity exercise is the best form of exercise to lose fat (6). However, due to the low total caloric expenditure, the total amount of fat used is small. As exercise intensity increases (up to about 75% of maximum heart rate), while the percentage of total calories derived from fat is smaller, that total amount of fat used is greater (34)."

[Empasis his, typos mine.]