Not sure why this tuned a little hostile but yes, eye.surgeon is correct that it is impossible to have negative miles per gallon.
But, we get the point ramajama is trying to make, so it is a moot issue on the technicalities of the math. He means that there will be a negative "effect" on mpg, assuming it is greater than zero to begin with, it will slowly decline at idle (but it can never be less than zero).
I will add that I lose about 1.5-2mpg here in the Northeast simply due to the changeover to winter fuel blends. So if I get 15mpg all summer, I get about 13.5mpg all winter, starting as soon as the fuel switch takes place. Idling to warm up the cars when it gets real cold makes this even worse.
I am not sure these modern engines waste any fuel starting, so I am not sure the start/stop function would waste fuel on restart that negates the savings.
My impression of the whole concept is that there is more to lose in terms of things going wrong and the costs to repair versus the savings in fuel. But, all newer technologies start off less than ideal and evolve over years and even decades, so perhaps some day in the future this concept will be perfected and everyone will be happy....