And Now, a Few Words From the Tourist Standing Next to You
He cut a hole in the back of an old film camera and stuck a slaved flash into it. By writing words onto transparent plastic and sliding the sheet into a slot near the film plane, the whole rig now works backwards and becomes a projector. By leaving the shutter open and slaving the rear flash, he can now project those words onto the subject of someone else's photo when their flash trips his flash.
His slave setup is a little clunky (kinda Steam Punk-ish, actually). But a standalone SB-800 also would work great -- without the camera-topping project box slave.
He's in it for the social commentary, but I can't help thinking how much fun this would be for sophomoric jokes, too. You know, if you were that kind of person...
(Thanks to everyone who sent this in -- www.JuliusvonBismark.com, via Gizmodo.)