Brain death usually only occurs after severe injury, either from trauma or from asphyxiation. Large portions of the brain ARE dead tissue; search for Terri Schiavo's MRI (or CT). Most of the skull is filled with fluid.

On the other hand, rarely comatose or persistent vegetative patients do wake up, or, more frighteningly, were awake and aware for years. I did some of my PhD research with a person who had a brain-stem stroke in 1980, who lived in a care center, and they didn't realize he was still awake and trying to communicate until 1992.