I could be wrong, but here's how I understand it so far, based on reading Rosedale's book and Stephan's blog. I welcome criticism!
Low carbers at normal body fat % don't have zero leptin, they just have lower leptin. Leptin levels cycle over the course of the day. Their brains are sensitive to circulating leptin. The amount of leptin present allows enough fat utilization for energy needs, and enough fat storage to replenish fat as needed. Everything stays in balance.
Low carbers at higher body fat % do NOT have low leptin levels, though the level may be quite a bit lower than when they were eating high carb. Their fat cells are still signaling that there's plenty of fat stored. A low carb diet will hopefully lower leptin levels enough to restore brain sensitivity to leptin. This, along with lower insulin levels, allows the individual to shift the balance from net fat storage to net fat loss.
A recent visitor here, awriter, believes that dietary manipulation cannot reverse leptin resistance in the hypothalamus. I haven't yet joined her yahoogroup to investigate further. However, Dr. Rosedale disagrees. He believes that dietary intervention effectively restores leptin sensitivity, though it will be slower and less effective as you get older (and presumably, more damaged).
Edited to add: Rosedale's diet is low carb, high fat (ideally from nuts, avocados, fish, olives), and moderate protein (somewhat lower than Mark's recs).