The Coup de Grace reads as a automatic critical to me.
Notice that it has no Attack line. Which would normally be,
"Attack vs Defense"
It only has a "Hit:" line, which always tells damage. In this case, the damage is automatically a critical of whatever attack chosen.
On the next page over (PH 289) the Coup de grace is listed again, in the table for standard actions and the description for it simply reads "Make a critcial hit against a helpless enemy" Meaning the action itself (taking a Coup De Grace) results in a critical. There is no attack necessary.
Compare for example "Basic Attack" which is listed above in the column and actually tells you to make an attack.
Note that a unconscious (or helpless) character is not merely prone, as in laying on the ground trying to avoid blows, he is simply lying on the ground motionless, helpless. He is no harder to hit than a door, a table, or any other static object.