Just kidding. In fact, it's the only approach that makes sense. I've advocated it for a long time. Recently, the Spellings Education Department apparently came to its senses and announced that the growth model will be used in 10 states to determine Adequate Yearly Progress (AYP). This is definitely a step in the right direction.
Jenny D points out some research that shows that measuring improvement works quite well. Go to her post for more info.