It'd be neat if you could hook up an iSight, point it backwards, and have a "live" image as your desktop, showing whatever's behind the computer.
Of course, it's even easier to fake a picture of:
Take 2 pictures of your desk - 1 with the laptop there, 1 without. Make sure the camera does not move AT ALL between photos (a tripod helps).
In photoshop, layer the two pictures, the laptop image over the non-laptop image.
Use the erase tool to remove whatever was REALLY showing in the laptop's screen area, allowing the other picture to show through.