Just a Suggestion
I have an idea for those that can't get the shirt in their size. How about you send in a shirt of your own, and Threadless will print it up for you? It could either be an extra donation on your part, or it could serve as a discount toward the price of the tee. Either way, it would solve the problom of Threadless being out of stock on tees (though they might start running out of ink...). Just a thought.
By the way, I love that they're doing all this :)