Richard Simmons.
...
Nah, really, lol. When the cold matter enters your mouth (or the back of your mouth), your body responds to the sudden drop in temperature by constricting your blood vessels to increase blood flow and minimize heat loss. The blood vessels then stop being constricted and this release is interpreted by the brain as pain, giving you the unique sensation. Hope this helps.
http://en.wikipedia.org/wiki/Brain_freeze

The reaction can be sometimes triggered within a few seconds after a very cold substance consumed comes into contact with the roof of the mouth. The body's response to cold environments is to vasoconstrict the peripheral vasculature (to reduce the diameter of blood vessels). This vasoconstriction is in place to reduce blood flow to the area, and thus minimize heat loss to keep warmth in the body. After vasoconstriction, they return to normal status and artery size results in massive dilation (vasodilation) of the arteries that supply the palate (descending palatine arteries). The nerves in the region of the palate (greater and lesser palatine nerves) sense this as pain and transmit the sensation of this pain back to the trigeminal ganglia. This results in pain that is referred to the forehead and below the orbit, other regions from which the trigeminal nerve receives sensation (This phenomenon is partially similar to the referred pain that is present in the left arm when someone is having a myocardial infarction). A similar effect occurs when one takes a prescription vasodilator, such as Nitroglycerin or Viagra. It is a stabbing or aching type of pain that usually recedes within 10–20 seconds after its onset, but sometimes 30–60 seconds, and can persist for up to five minutes in rare cases. The pain is usually located in the midfrontal area, but can be unilateral in the temporal, frontal, or retro-orbital regions :)

OMG that is an awesome question! I wish I knew the answer. I'll be watching to see if someone does!