One day you might have this thought: "Oh look! Someone lost their USB flash drive and now it's mine!" And sometime later your system might become "owned," probably after two seconds after you plug it into your computer.
Sound far-fetched? Well it's certainly an enticing lure. After all, finding anyone's lost portable storage device (whether it be a camera, flash drive, MP3 player, etc.) is an opportunity to see what's on it. Of course to do that you have to plug it in to your PC, and that's very dangerous.
This sort of scam is apparently already being played on people in London, apparently. You can read a little bit about it over at The Register.