More common then people realize. Coma victims have often experienced the afterlife and NDE. It's actually quite a common occurance. The problem is that if they try and tell doctors or something the doctors credit it to some chemical brain reaction or something. But to have an OBE or NDE during a coma... well... many, many books are written about such experiences.
As was stated above, the near death experience is more common than thought. While there are numerous books on the subject there are also quite a few websites. I have listed a few below for anyone who is interested in the subject.
http://www.iands.org/
http://www.near-death.com/
http://www.mindspring.com/~scottr/end.html
Very best,
http://www.gazette.net/200306/frederick/news/143101-1.html
Brunswick couple finds blessings in recent spate of health challenges "Just about the only thing Bob does recall is that sometime during his coma he "saw the angels" his mother, her parents and many relatives on his father's side."