Apparently, it is legal in several states, including California, not that I think it is ethical. That's why, in a previous post a whole lotta days ago, I dared suggest that we check to see what the laws are in our respective states, and work to change them(and I got soundly blasted for such a revolutionary concept). I also believe we need to understand what the current thoughts of ethicists are, because they have a lot of influence. We need to look long and hard at what the bioethicists have proclaimed as "ethical". For example, bioethicsts believe that it is not ethical to withhold a heart transplant from a prisoner on death row if his medical condition is such that he might need one. We are looking at a major cultural change (that's a news flash, eh?).
It's also why we need to have an advance medical directive, make it as specific as possible, and review it every now and again. It's also a good idea to have a durable power of attorney in place to allow a trusted loved one to make those difficult medical decisions for you should you be incapacitated. In my case, I don't want to be starved and dehydrated to death, but if I've suffered major brain damage, neither do I want a feeding tube installed so that I can be mainted for 20 or 30 years. No thanks.
Medical science has advanced greatly over the past decades to the point where people who would have died can now be rescucitated and/or maintained. That includes people who have had no pulse for many minutes, and have suffered serious, irreversible brain damage. Medical technology can get the heart going again in many cases, but I question whether or not that is always wise. I know that if I've been without a pulse for 10 minutes or so, I don't want anyone anywhere near me with those jumper cables. After 5 or 6 minutes w/o O2, the brain cells start to die, and they aren't going to grow back. If my cerebrum is gone, then I'm gone, never mind if some hotshot EMT gets my heart going again.
Regarding your point about brain cells: Each organ has unique properties. The liver, for example, has a remarkable ability to regenerate. Only the skin shares this special property. A young, healthy person can even donate half his liver to a biological match, recover, and lead a full and normal life. The lungs, too, while completely formed at birth (all bronchi are present), can heal after years of abusive smoking and toxicant inhalation.
One of the important features of the brain is its functional plasticity. It was once thought that functions were restricted to specific regions of the brain. We now know that this is not true. Taken in light of the fact that we only use about 10% of our brains, this makes sense.
While ultimately only you can express in an advance directive your wishes for treatment in a tragedy like you described, I would urge you to consider these facts. Your life is valuable, and you, along with all of us, have special meaning and purpose. I would not want this view to become the default, however, in the absense of a written directive; life is too precious, and this decision too important to err on the side of the irreversible.
When did it become legal in California?
It must've been since 1997?
See the Wendland case - a mirror of Terri's - and the judges decision:
http://www.angelfire.com/ca7/robertsangels/WendlandToLive.html