The Fat Burning Zone is often asked about. Here are some articles that give you plenty of technical data behind why you need to work out HARD.
http://www.pponline.co.uk/encyc/0895.htm
http://findarticles.com/p/articles/mi_m0675/is_5_20/ai_92840201
The short of it all...the lower intensity zones aren't for people trying to lose weight. They have their place for athletes and those training or specific events. Lower intensity is also good for a RECOVERY workout.
Those who are trying to lose weight just need to bring to the gym. Focus your efforts on interval training, high intensity, and HIIT training.
http://phit-n-phat.com/blogs/phit-n-phat/archive/2007/10/20/fat-burning-what-you-need-to-know.aspx
http://phit-n-phat.com/blogs/phit-n-phat/archive/2007/10/17/all-about-the-cardio.aspx
http://phit-n-phat.com/blogs/phit-n-phat/archive/2008/01/14/why-track-cardio-to-burn-fat.aspx
http://phit-n-phat.com/blogs/phit-n-phat/archive/2008/01/09/cardio-that-burns-fat.aspx