Here's a photo of my desk at work, showing off my Mac with the transparent screen option. (I cleaned up the desk a bit before taking this shot.)
And here's a remarkable collection of photos of computers with transparent screens:
Sure, this is silly. But it's also a lot of fun.
The real question is, Why are almost all of these transparent screens running on Macs? One possiblity is that Macintosh fans would be more likely to do something like this. Another is that Macs still have some mystique that contributes to the illusion; it's almost believable that a Mac would have a transparent screen option.