I heard this one tonight while watching a rerun of It's Always Sunny in Philadelphia. Partway through the episode, Danny DeVito brings in a bunch of scraggly homeless men and pays them cheap to wear sandwich boards. He calls it hobovertising.
The idea of hobovertising isn't a new one among jokesters. Like selling homes near the airport to the deaf and getting Jehovah's Witnesses to deliver the mail, getting the homeless people who already hold up cardboard signs to hold up advertisements is one of those comedic ideas that pops up every now and then.
Someone thought hobovertising was awesome enough to actually merit its own fake Web site, Hobovertise.com.