At a minimum of $30,000 per sponsor, and given that the t-shirts won't happen until all the spaces are full, the spaces are probably going to stay empty. Especially since they're only producing the T-shirts in size "medium" and making them available to US residents.
A cooler idea would be to have a site that gives away t-shirts with company logos on them where the company pays for the t-shirts, shipping and a bit to cover the site's costs. That way e.g. Ford or Coca-Cola could "seed" 1000 random people around the world with shirts carrying what is in essence their ad. Each company would be exclusive to one "batch" of t-shirts, where the batch size could be 1,000, 2,000 and so on. Doesn't exist, but it probably has a better chance than the million t-shirt idea...