You know what that building across the street from you needs? A graffiti tag, from your iPhone. Here's how you do it.
F.A.T figured out the trick, which involves little more than custom Fat Tag software, AV cables and a projector. Detailed instructions can be found here. It's (obviously) just a projected image, so for better or worse you won't do any permanent damage to your neighbour's property. But it'll be fun to see how long it takes Old Man Dithersbottom to realise that. [F.A.T. via Academic Bulimia