Much discussion over the results of study of the effect in NYC of posting the calories in restaurant foods. In short, no significant impact on the food chosen. Kevin Drum and Ezra Klein aslo link to it.
I'd caution people to relax and think long term. I remember when the health warning was added to cigarette packs. It took a long time, but that was one step in establishing a social consensus against smoking. That, the consensus, is what is effective in changing behavior.