Is it as simple as 1 pound = 3500 calories for weight loss?

A lot of diet books, health columns, magazines, etc. tout the basic rule that to lose a pound of body weight, you have to burn or "not eat" 3500 calories. That is - it takes a deficit of 3500 calories to lose a pound of body fat.  The SL was watching the Biggest Loser the other day - and the SL saw Jillian pounding this idea into the contestants.  So, is it that simple?  Well - yes and no.  A pound of body fat does contain 3500 calories worth of fat, so in theory if you burn an extra 3500 calories through exercise, you will lose 1 pound. However, your body is a bit more complicated. A bout of exercise will burn calories, and it will also boost your metabolism ever so slightly the rest of the day (you get some bonus burn). Also, other factors affect your metabolism - whether you feel hot or cold, whether you got enough sleep, if you ate breakfast, if you ate too much salt (water weight), etc.
Also, your body is good at compensating for changes. Normal weight folks who overeat for a day or two mysteriously process food without gaining weight (this doesn't last forever - eventually the body adjusts and you start gaining). People who start an exercise routine may inadvertently sit around more the rest of the day - canceling out the exercise efforts. Losing weight is just as much a mental battle as much as a physical battle with yourself. You have to outsmart your own body - and that's the tough part. Counting calories in and calories out can be simultaneously over-obsessive and inadequate at the same time!