People have been talking about this on adlist, and I've received a bunch of mails containing the images of Ian Hart's ford Mustang concept billboards, a few mails incorrectly stating that these ads were real and running right now because that's what they read on some random adblog out there.
Not the case.
(Here are posts that don't state it's a concept only: #1, #2, #3)

On Ian's concept page he writes:

"Constructed from GE Lexan semi-transparent resin, the billboard accurately blurs the scene behind it regardless of day , weather or season"

This seems like a great idea at first glance, but you know it's impossible....

Does anyone wanna break to Ian Hart that regular old billboards can't look that way? Since they're stabilized on the back and/or have another billboard there.

Back of billboard shot by ten safe frogs.

To be able to do it, Ian would have to either go the "transparent image" route, like those Amnesty ads did. Which wouldn't be as much fun.

Alternatively he could build special billboards with frames around the plastic. But in that size, I suggest he calculates carefully where to leave air holes, as wind has a way of doing nasty stuff - and we had to build a few test boards back when I did transparent billboards for Sonox.com years ago so that they wouldn't snap in half or fall down. Ian's concept boards look like those giant 90-sheet billboards too, they need a lot more stabilizing than my wee posters did.

Other than the constraints of reality, top idea mate.