Three people from Jackass tried the milk thing...one person used regular milk, one had chocolate milk, and the last had strawberry (eep!) milk. Needless to say they all hurled, but one of them did finish the whole gallon...it was a girl beating two guys!
I also find the biological limitation of this feat to be very intriguing. I'm sure there is some freak of nature (like Kobiyashi) who has an enormous over-abundance of lactase that could do it given the training.
The cracker thing is also pretty dang impossible...I was just reading on the Guinness Book web site that some guy ate 3 giant cream crackers in like 35 seconds - hopefully he had some Pavlovian training in order to obtain the necessary saliva.
The only bag milk I've ever seen is the stuff they put in the milk machines at college...