I've always been intrigued by this bet, but for several reasons not undertaken it or wagered against it. The thing that really puts the security on the gallon of milk in an hour bet is the fact that it is, in fact milk, and not as much that there is a gallon of it.

Milk is a horribly hard substance to digest. The fact that it is a gallon of it helps because there is an association with Americans with milk in a gallon. (If you'd ask people what they thought when you said gallon, they'd probably say gas or milk). Other items in place of this bet:
  • A gallon of soda(or pop, whatever you call it) - A gallon of soda is fairly indigestible as well. You'd have to drink it fairly slowly, or flatten it first.
  • A gallon of water - This one a person would have to be trained to drink lots of water, and even then, they'd probably throw up (as the body can have tough time with plain water). There is only so much water one's body can take (and you can be pretty sure it's under a gallon). This however, would be closer than milk.
  • A gallon of chocolate milk - This is a safer bet, since chocolate milk is heavier (although tastier) than regular milk. It's still milk, however, and would still be tough to do.
  • A gallon of skim milk - Watch out. The closest I've ever seen anyone come is with a gallon of skim milk. You want them to drink the heaviest milk they will agree to, if you are the one proposing the bet.
  • A gallon of beer - If you can find someone that will buy me good beer, I'll take it. Hell, I'd love to fail on this one.
  • A gallon of Gatorade - This one is the true nail biter. If you want to process the entire liquid in an hour, a mildly sugary substance (or something specially formulated to be so, like Gatorade), is something that they might be actually able to win. The body metabolizes those sorts of things fast, compared to other substances.

As a testament to the evils of milk drinking, a friend of mine did not drink in social situations because of a general objection to it. However, at parties, when people were doing drinking games, she would take shots of milk (at games like Asshole, or whatever people call it), instead of a required shot or liquor or beer. The next morning, she'd be more wasted (headache, feeling horrible, throwing up) than anyone drinking the alcohol. At least you can build up a tolerance to beer...