Given that large tax breaks already exist for buying SUVs, another tax break for buying a more fuel efficient one sounds like a good idea to me, especially as the saving in gas from better mileage will probably return a large chunk of the extra purchase cost over time as well so you actually end up being paid to drive the thing - relative to old model SUVs.

Not only that but I think it would give the car manufacturers more incentive to try new technologies. Once a new "breed" of SUV hits the market there will be a fairly rapid evolution resulting in better & better designs. One danger is public backlash from poor designs that come with many major problems (unreliable trucks/cars that turn people off).