Has anyone ever sucessfully brought back a cryogenicaly frozen person?
Actually the Massachusetts General Hospital have successfully revived pigs that the "killed" through up to 90% blood loss (leading to a total lack of brain activity and heart beat) by bringing them down to extremely low temperatures for about three hours or so, then restoring their blood and starting their heart beat up again. According to them they managed to perform this procedure over 200 times, with about a 90% success rate of revival. Before them a University of Pittsburgh Research lab performed a similar thing on dogs. (DARPA is also doing current research into similar topics, but haven't released any results that I know of yet). It's only a matter of time until we have the technology that could successfully a person.
Of course even assuming we had the technology to successfully revive people you run into all sorts of other problems, notably:
1) Most cryogenically preserved people have bigger problems that caused their death, such as old age failure, disease, etc. that we can't cure right now.
2) You can't exactly test things on human subjects, even volunteers, since voluntary anesthesia is still illegal.
3) In addition to making testing difficult, this also means that those who are dying of disease are forced to the point of critical failure before we can freeze them. Even if we developed a cure for something like cancer, it's going to be much more difficult to "save" a patent who's body consists of basically one massive tumor than it would be if they were frozen in the early (but still untreatable at the time) stages.
4) If you attempt to revive someone and it fails, is that murder? What about the contract that you sign with them, would it be a form of contract breach? How safe does the method have to be before it's "worth the risk" for the patient? 50%? 90%? 99.9999%?