According to this article this move on Pepsico's part is going to initiate a new marketing war in which all manner of food products will feature similar labels, in the hopes of garnering increased sales from the green-friendly demographic. What brand of overpriced sugar water do you want to buy -- if the nutritional elements are more-or-less the same, they're hoping you'll go with the one that sports a lower carbon emission label.
Once again, I'm really big into positive change and social responsibility, but this seems like a cheap marketing ploy to me. There's no context -- clearly if there's a brand that produces less carbon emissions that's a good thing -- but really, how is the layman to know the environmental impact of five or six grams of carbon? I think it's going to be an excuse to jack up the already-inflated prices on consumer goods in the interest of pretending to be "green". Carbon is carbon, whether it's 50 grams or 30 grams.