Decisions on when to maintain and when to remove artificial life support are difficult at all times. Ideally, the decision to remove artificial life support is made reluctantly but unaimously by all parties involved, including spouse, parents, siblings, and adult children. When there is disagreement, I believe that a hierarchy should prevail: First the patient, of course. If the patient is not able to express his wishes, and has left no written directive, then the decision belongs next to the spouse. That's the nature of marriage, as hard as that can be for parents to bear.
The case of Terri Schiavo is not as clear-cut as that, however. Although Michael Schiavo is legally Terri's husband, he has lived for years with another woman, and indeed has two children by her. It's hard not to sympathize with him, and many will see nothing wrong with his actions. Nevertheless, by so doing he has forfeited all moral and ethical, if not legal, rights as Terri's husband, and should leave all decisions concerning her care to her parents, who have demonstarted that they truly will stick by Terri, "for better, for worse, for richer, for poorer, in sickness, and in health, to love and to cherish, till death do us part."