From what I have heard is that you body heat begins to curdle the large amount of milk in your system, and your body gives you no choice but to throw it up.
I love milk, I drink 1 to 2 gallons a week, and there is no way in hell I could stomach and entire gallon of whole milk in 60 minutes.
Like I said this radio station has done it for years and have yet to get one person to finish.
I'm pretty sure that most people don't have a stomach that can fit a gallon of fluid which is most likely the reason that it can't be done.