Sometime back I ran across an interesting dicussion about these on a site seriously dedicated to helping folks get better mileage. The discussion on the site was very enlightening and informative. I checked out the company's site and found some good information about how these work, they appear to have some merit for consideration. Just for my own interest I posted a message on the site asking if anyone had experience using the product on a pick up. No answers yet as of posting about two weeks ago. I thought I'd post this in case anyone was interested.
As an FYI, the site also had a discussion with photos and instructions on how to make your own out of CD disks.
Look at an evo X, has the same thing. it's called a vortex generator. on the evo it helps funnel the air into the rear spoiler to provide more downforce, while providing additional stability.