Interesting article in The Boston Globe which profiles researchers who suggest that variation in gut flora (the mix of bacteria) might be the cause of differences in body weight. Interesting fact: there are an order of magnitude more bacteria in your gut than cells in your body. Also, to my knowledge (hearing this from a microbiologist last year) this gut ecosystem hasn't really been replicated in the lab, so that kind sucks for a scientific understanding beyond description.

Update: Aetiology has much more.