Yes, they stack
Because they are different types of bonuses (racial, and untyped).
Remember also that untyped bonuses always stack unless they come from the same game element.
That is, if your halfling rogue is within 5 squares of an ArdentDDI ally with Wisdom 16 and the
Mantle of Clarity feature, he takes an additional +3 (untyped) bonus to his AC against opportunity attacks (totaling a +10).
But, if your character is within 5 squares of two allied Ardents, one with Wisdom 16 and one with Wisdom 19, he would receive "only" an additional +4 to AC vs. opportunity attacks, because the two
Mantles of Clarity don't stack and only the strongest is used.
Mantle of Clarity
You and each ally within 5 squares of you gain a bonus to all defenses against opportunity attacks. The bonus equals your Wisdom modifier. In addition...