Casein is a protein that is found in milk. Groups of casein normally float around in milk without bonding. These groupings have a negative charge. The acidity of the Red Bull causes the negative charges on the casein groupings to become neutralized. Instead of pushing each other apart, the casein starts to clump together causing the milk to curdle and form solid matter.
Original link: http://www.youtube.com/watch?v=WW64NUKnHEM