Depends on what you're marketing... what if you're marketing a funny video website like break.com, then it has some serious potential. If you're marketing the next Ipod, I doubt seriously anyone is going to be like, "wow, what a great company".
I just cannot see this idea working. There is a reason why companies hire models to wear their company t-shirts and not homeless people.
One of the best ideas ever, why didn't I think of it. But why stop at homeless people? There are so many people that won't turn down a free shirt. Thank you for the idea.