Great recommended reading from Keith at Theory to Practice discussing the age-old argument of caloric balance. Weight loss or maintenance is much more that balancing or reducing calories in versus calories expended.
Condensed bottom line: When insulin levels are high resulting from carbohydrate intake, this signals the cells to store fat. Lack of insulin in the blood stream frees fat from the cells to be used as energy. In extreme carbohydrate-restriction (extreme using the Standard American Diet as a baseline), the body goes into ketosis which primarily uses stored fat as an energy source vs. glucose.
Although each and everyone has their own disposition how well fat is either stored or used for energy, nutrition plays a huge part for the common man.
So, regarding the Paleo Diet, it is not as severe as Atkins regarding carbohydrate restriction, but most carbodydrates seem to be complex carbs that are found naturally in vegetables; go hog wild. There is provision for simpler carbohydrates in fruits - which is to be consumed with some moderation.