I found this article today on Google News:
http://www.whatcar.com/news-article.aspx?NA=228254
A British university has published a study ranking a number of new cars according to their overall environmental impact. The study considered emissions, energy usage and end-of-life disposal costs. Although the weights given to the different criteria were somewhat arbitrary, it's good I think that they looked at factors beyond straight gas mileage.
The smart coupe and cabriolet placed first and second; the Toyota Prius finished 12th.