Is it possible to be (briefly) aware you had been decapitated?
According to this, in rats it takes about 17 seconds after decapitation for the EEG to become iso-electric. But there is no known correlation between EEG and consciousness. Also at 50-80 seconds after decapitation, EEG being iso-electric, a very slow, late wave appears on the EEG record.

The same article concludes that it takes about 3-4 seconds after decapitation for the animal to lose conscience and perceive no stress and pain.

Also, dr. Harold Hilman states in An unnatural way to die article from october 1983 issue of New Scientist, page 277 that:


  Consciousness is probably lost within 2-3 seconds, due to a rapid fall of intracranial perfusion of blood.