I've been completely obsessed with this idea for almost a year, and have tried it many times with varying success. The idea is that you take a picture of what is behind your monitor, and use it as your wallpaper, giving the appearance of a see-through monitor. It's not as easy as it looks. Here are some mind-blowing images from Madbidouille:
(Click image for larger view)
If you've got a setup like this, or end up doing it after seeing these, add the link to your image in the comments. I'd love to see what else is out there.
Wallpaper City Guide Barcelona 2011
The Little Mac Book, Snow Leopard Edition
Macs For Dummies