Wikipedia says,
Decapitation is invariably fatal, as brain death occurs within seconds to minutes without the support of the organism's body. There is no way to provide life support for a severed head with current medical techniques.
An even more gruesome issue is whether a decapitated head retains consciousness after separation from the body. The issue has been debated many times {1}, especially in the context of whether beheading is or is not a humane form of execution (see below). No definitive answer has ever emerged. Many have argued that loss of consciousness would be virtually instantaneous, either as a result of the massive drop in cerebral blood pressure, or because of the impact of the severing implement. Nevertheless, anecdotal evidence, of varying degrees of credibility, has circulated for centuries that victims may, under some circumstances, retain consciousness for at least a few seconds.
The body of a decapitated chicken was once known to live for an additional 18 months. (See Mike the Headless Chicken).
Later went on to become John Prescott.
Full story can be found here.
http://en.wikipedia.org/wiki/Decapitation