http://news.bbc.co.uk/2/hi/health/7452547.stm

"Researchers may have found a way to predict whether severely brain-damaged patients will regain consciousness.

A part of the brain which can stay active even in severely brain-damaged patients could offer a clue about the chances of recovery, they claim.

The Belgian team told a conference that activity within a "default network" in the brain appears to match the level of consciousness of the patient.

Some believe the default network is associated with daydreaming.
The findings were reported in New Scientist magazine."

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The article goes on to say that the level of daydreaming is uniquely related to the state of the patient's brain, with a brain-dead person, of course, showing no activity at all.

The patients in a coma or persistent vegetative state have activity in this area at a lesser level than someone who is simply "minimally conscious" might have, but more than the "brain-dead" person might have.

I recently read an article about a woman who awakened from a "brain-dead" state, as her family was being consulted on organ donation, so this seems to be an important breakthrough in understanding how close a person might actually be to brain death -- but how would it account for the stories of miraculous recovery, such as in this woman's case? She had supposedly been "brain dead" for a long time.