Thanks to everyone for the great comments, this has been an interesting discussion.
I received a very kind e-mail response from Dr. Lindeberg, in which he told me that his group didn't measure leptin levels in his paleolithic pig study because it would have required special reagents. He also sent me two very interesting papers, both hot off the presses.
The first paper shows that glycosylation (bound sugars) of the leptin receptor is required for normal leptin binding. One of the molecules they use to probe the function of the leptin receptor is our good friend wheat germ agglutinin (WGA), a lectin found in wheat, barley and rye. They used WGA to specifically block leptin binding at the receptor.
This fits in very nicely with the hypothesis that grain lectins cause leptin resistance. If WGA gets into the bloodstream, which it appears to, it has the ability to bind leptin receptors and block leptin binding. It doesn't take much imagination to see how this could cause leptin resistance.
One caveat is that they used a high concentration of WGA in the study; 10 ug/mL was the lowest concentration they used. I can't imagine that concentration is possible in an actual human body. However, the paper doesn't explore the lower limit of WGA's ability to block leptin binding. At the lowest concentration used, it blocked 50% of the leptin from binding. It's possible that much smaller amounts could still have a significant effect.
The second paper Dr. Lindeberg sent me was on the soy isoflavone genistein. Here's the executive summary: it's bad. Unless you are a man who really wants to embrace his feminine side. It gets into all tissues and effectively activates the estrogen receptor in mice. It shrinks the prostate just like administering estrogen. It also passes into pups through the mothers' milk at levels high enough to activate their estrogen receptors. All this from the same amount of genistein you can get by eating a meal of soy.
The bad news doesn't stop there. Fermentation doesn't break it down. Miso, tempeh and natto actually have more genistein than non-fermented soy. Sigh...