My first 5 minute attempt at a transparent screen
I had to try this. Some observations:
The trick seems to be to step back a little and zoom so that your screen fills the viewfinder, then close the lid and take a shot [after zooming out again], import it and try to match the color tone, exposure, brightness, etc. of the surroundings. Then set the picture as desktop picture. The next step is to reposition the powerbook and the camera until it matches.
Or at least, that's a way that seems to work. This is far from perfect, but it's what I could achieve in five minutes.
Update: After having reviewed most of the attempts in the pool, it seems to me as if getting the colors and brightness of the desktop picture right is more important for the end result as [than!] getting the alignment right. There are several examples of well-aligned shots, where the poor color and brightness match ruins the result.