Mercy killing, Yes or No

Read a post on CNN that Terri Schiavo's widower had remarried. For those of you who don't know who she is, try to google up.

Terri Schiavo died in March after her feeding tube was removed. She had suffered irreversible brain damage after collapsing at age 26 in 1990. An autopsy supported Michael Schiavo's contention that she was in a persistent vegetative state with no consciousness and no hope of recovery.
(Source : CNN.com)

Now, here's the big big question. What should be done in such a case? There's obviously going to be an argument, like there was in this case between family members, and most of which does not depend on the nature or closeness of any individual, but an individual's assessment of the present situation, both in medical and practical terms. For example, if a doctor told Terri's husband that she would never recover again, wouldn't he be entitled to ask him to not let her remain alive?

On the other hand, speak of medical miracles. What if some miracle or some groundbreaking technology enables the patient to live on further? At a time when the patient is not making a decision, is it fair for other people to just let her die?

I have always been confused on this issue myself, right since my school days, its a tough one.. what do you say?