I've seen those at Goth parties (sometimes advertising another Goth party). I was always wondering if they had made these themselves.
Apparently not (unless those people sell 'em on thinkgeek these days).
I'm guessing the message from remote to badge is not cryptographically authenticated. So it can't be that hard to program other people's badges, remotely and covertly... how long would it take them to notice?
"point the IR remote at the back of the LED badge, push one of the buttons, and a new message is set to display on the scrolling badge."
Bit of a challenge ;)