I know this is just a single data point, but they've already instituted a law like this within King County. It has certainly changed the way I order when I'm out. Accuracy of the numbers notwithstanding, you can clearly see huge differences in calorie counts between different menu items, and I've personally steered clear of some of the more daunting ones that I might have otherwise ordered.

Once again, that's just me, and it doesn't really speak to the accuracy of the figures.