The trick to reading a playing card that is facing away from the camera is picking up light that is reflected off of a surface behind the card. "In the card experiment, the camera cannot see the card directly, but it can see the surface of the book [behind the card]; the light from the projector bounces off the card, then bounces off the book and hits the camera," said Sen.Link (via We Make Money Not Art)
When the projector shines on a red part of the card, like the heart of the suit, the light gets a red tint. "The camera observes it and our algorithm determines that the projector saw something red at that position," said Sen. When the camera shines on a blue part of the card, the light is blue. "In this manner, we put together the projector image pixel-by-pixel and can see the card," he said.
Update: Keith sez, "this link goes to the researchers' website, where they have a video demonstration of their technique (including the card trick at the end)."
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.