The Depressed Metabolism blog includes a wonderful introduction to cryonics that leads in from the use of other forms of metabolic arrest in medicine. It's far better than the one I put up some years back at the Longevity Meme. You should head on over and take a look:
Ethically, what is the correct thing to do when medicine encounters a difficult problem? Stablize the patient until a solution can be found? Or throw people away like garbage? Centuries from now, historians may marvel at the shortsightedness and rationalizations used to sanction the unnecessary death of millions.
Cryonics does not involve the freezing of dead people. Cryonics involves placing critically ill patients that cannot be treated with contemporary medical technologies in a state of long-term low temperature care to preserve the person until a time when treatments might be available. Similar to such common medical practices as general anesthesia and hypothermic circulatory arrest, cryonics does not require a fundamental paradigm shift in how conventional medicine thinks about biology, physiology, and brain function. Although current cryopreservation methods are not reversible, under ideal circumstances the fine structure that encodes a person’s personality is likely to be preserved. Complete proof of reversible vitrification of human beings would be sufficient, but is not necessary, for acceptance of cryonics as a form of long-term critical care medicine. The current alternative is death; or for persons who are at risk of suffering extensive brain injury, loss of personhood.
For very old and fragile patients, meaningful resuscitation would require reversal of the aging process. Obviously, the objective of cryonics is not to resuscitate patients in a debilitated and compromised condition, but to rejuvenate the patient. Ongoing research in fields such as biogerontology, nanomedicine, and synthetic biology inspire optimism that such treatment will be available in the future. The fortunate thing for cryonics patients is that even if fundamental breakthroughs in these fields will be the result of long and painstaking research, the cold temperatures allow them time - a lot of time.