I found this page on Dual Photography on Slashdot. This group of researchers using something called Helmholtz Reciprocity to virtually exchange a camera with a light source to produce an image from the perspective of the light source.

As in, you take a picture of something that is lit from the right side, you can then process the image and generate a version of the image from the perspective of the light source that was on the right. Doesn't sound all that big or cool until you see what they do with a playing card and a book. There is a bittorrent for the big video on their website that I totally suggest you download and watch. (Download BitTorrent, it is a peer-to-peer service that lets you virtually host content across lots and lots of machines so the distribution of the content doesn't have to come from just the original source and therefore everyone can get to it faster.)

The last example they show is a camera that is looking at the back of a playing card with a book in the background. The light source is off to the right and from the perspective of the light source, you can see the face of the card. Based on their algorithm, you can take a picture of the back of the card against the background of the book and generate an image that shows the face of the card (because the light source can see the card). So, they are literally being able to determine what is around a corner they cannot see by capturing small differences in the light that bounces off of the playing card, then off of the book and at the camera. It's crazy!

I know I'm babbling and what I just said probably doesn't make a lot of sense... just go watch the video! You'll understand!