According to CNN, the rules work by dividing SUV and light trucks into six classes by weight and type. So, the car companies have to improve the fuel economy of an 5 ton SUV, but they could increase its weight to 10 tons and have an easier target The fleet average could actually get worse if SUVs continue to increase in weight.
I've also heard what Kate says, that the fuel economy standards were not imposed; perhaps the very largest SUVs and "light" trucks aren't covered by this.
--Walter
PS I've decided that an occasional commute in an Abrams Tank would get me more respect on the road than my Civic. Especially when you bring the cannon to bear on an giant pickup truck's rear window.