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Well, I suspect it's a bit of both. Unlike the two examples kach22i put up, these 2010s and 2011s have a noticibly rising line to the crease, not just following parallel to the ground. I'd suspect they're trying to move air into the low pressure zone behind the rear window. The only 2010 I saw with this was the Prius, but now it's appearing everywhere including the new CR-Z (just saw my first one in the parking lot at work today).
I tried to look at highway MPG changes, but while every car I've noted got better MPG with the introduction of the upward wedge, they also all had changes in engines that probably played as big or a bigger role. The Prius, which doesn't appear to have had a change in displacement, had other changes which resulted in an increase in highway and city mpg by 3mpg.
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