I have just been listening to one of Dr Joel Fuhrman's audio phone-ins on voiceamerica:

11/29/06
“Food Addiction: The Cause of Obesity”
Download MP3

At 34 minutes, he talks about a "metabolic setpoint", where your weight gets stuck at an artificially high level. It is also explained at Body and Fitness here, and I quote:

"When a person has maintained a particular weight for a long time, there seems to be some latitude in eating that is not present for a person who has recently lost 20 to 30 pounds. This appears to be explained by the theory of metabolic set point, or the weight at which the body maintains homeostasis or balance. It seems to be determined by the ability of the body to burn fuels and the general level of metabolic fitness. When the metabolic set point and the actual weight are the same, balance prevails. Dieting, particularly fasting or a crash diet, disrupts this balance. If weight is lost very rapidly - say, 170 pounds down to 130 pounds in six weeks - the set point may still be up around 160 or so, creating a tremendous tension between actual weight and set point. There is a tendency for the weight to be drawn back up again to restore balance, even though eating is minimal. That explains the discouragement that many dieters experience as they reach plateaus where they seem hung up for long periods of time. I never suggest rapid weight loss without preparing him or her for frustration, hardship, and disappointment, and I do not advocate more than a two-pound weight loss pre week. By bringing weight down slowly, the metabolic set point has a chance to adjust, and a quick return of the excess weight is not as likely. A slow weight loss, with concurrent change in lifestyle and eating habits, is the only way for fundamental and lasting weight maintenance."

Dr Fuhrman says in that audio above that tests were carried out on people losing weight who had been overweight for a long time, and their metabolic setpoints were stuck at an artificially high level for 6-9 months then it drops. So perhaps this is another reason why we increase our weights after an extended fast, and possibly the reason that people plateau during the fast, and after the fast.