I found this explanation for a somewhat similar shot on About.com:
"Comments from a photographer: I would like to explain the "transparent man" phenomenon. The result of the photo is caused by sync issues between the camera's shutter duration and flash timing. Very likely, the shutter on the camera remained open even after the flash went off. In this scenario, once the flash went off, the couple very likely disengaged from their pose, revealing the previously obscured backgroud to the camera with its shutter still opened. In this instant, the camera captured very briefly, the area behind the man and also theorectically, there should be another image of the man leaving the posing position."
So... this sort of makes sense, but if this was the case, wouldn't he be all blurry as he moved out of the shot?