Hikerdon raises a very good point, a person in cardiac arrest (most adult arrests are cardiac in nature) needs defibrilation within no more than 8 minutes. CPR may raise this to about 12 minutes, if you're lucky. So unless you have quick access to a defib, CPR will not revive the person.

If it is respiratory arrest however, the chances of CPR reviving the person are much better. In drowning victims, casualties with a head injury or young children, CPR may well revive the person.
The same is true for hypothermia, but the casualty must be SLOWLY returned to the normal temperature with constant CPR being performed.

Chris