Excerpt from Physicist Michio Kaku 4 part documentary on "Time" Michio investigates claims of time dilation under the influence of Adrenaline and other drugs.
IMHO, it's all about our perception. The brain, under certain stressful situations, is more active allowing more information to be processed. To put it another way, under normal conditions, we see the world at ~32 frames per second... Under life threatening situations, this is increased giving the illusion of time slowing.