It would be worthwhile to take the time to read the lead article in the 17 Aug 09 edition of Time Magazine.  The author makes a lot of good points about the fact that the "Aerobics" fad promoted by many as a weight loss strategy is pretty much useless.  But what he really should be talking about is that "Strength Training", by maintaining and sometimes increasing muscle mass is the best strategy to utilize when you want to lose weight (read lose fat, maintain or increase muscle).  Small decreases in muscle mass which occur on a yearly basis, unless we do something to prevent it (read Strength Training) will set us up to get fat.  Sarcopenic Obesity, obesity related to the gradual loss of muscle mass and its metabolically functioning components is what it is called and it is what we are trying to prevent.  See the link to NEAT, Non Exercise Activity Thermogenesis which may be more important than exercise over the long term to prevent fat gain in the first place!!

 See Article of the Month Age Related Decline in Metabolic Rate!