You can't let the paladin and cleric find out? So they have noticed your flesh has fallen off and withered? Or the Paladin hasn't used Detect Evil which reveals evil in the area?
As to the question ... it would seem that Concentration is one of those skills that falls under many categories depending on your view point.
Definitions from SRD:
Strength measures your character’s muscle and physical power.
Dexterity measures hand-eye coordination, agility, reflexes, and balance.
Constitution represents your character’s health and stamina.
Intelligence determines how well your character learns and reasons.
Wisdom describes a character’s willpower, common sense, perception, and intuition.
Charisma measures a character’s force of personality, persuasiveness, personal magnetism, ability to lead, and physical attractiveness.
Each one of those definitions with the exception of Strength and Intelligence clearly defines a characteristic that would fit under Concentration. For example Dexterity contains balance and coordination. This would be the appropriate stat if you were concentrating on a tightrope wire high above a a canyon. Consitution has stamina which is good if you concentration after 24 hours of no sleep. Wisdom is willpower, the sheer ability to concentrate under duress. Charisma is force of personality, the ability to concentrate when everything weighs upon you.
So we have 4 possibilities here. You can remove Dexterity because most balance issues are catered for using the Balance skill. The spell-caster must use Balance on the tightrope and then Concetration to hold his balance.
You can remove Charisma from our list because force of personality boils down to willpower when it is internalised.
This leaves Wisdom and Constitution. Both have their merits. Constitution is for physical distraction, Wisdom for mental. The last test would be the usefulness of both traits.
How often is Concentration used and in what situations? In D&D more often than not, Concentration is used to avoid an AOO. The other situations I've seen them used is in Entangle spells, and not many other locations.
If you were to make a list of situations that required Concentration you will find that most of them are physical. Thus Constitution is the logical choice.