Bladed Step, one of the Knight's Weapon Specialization options, has the following text:
Benefit: After you use power strike with a heavy blade, you can immediately shift 1 square to a square adjacent to an enemy. You then gain combat advantage against that enemy until the end of your next turn.
Heroes of the Fallen Lands, pg 136
Obviously the shift itself is optional (from the use of the word "can"), but if you didn't want to change position, could you still gain combat advantage from this feature without moving? i.e. simply choose one enemy you're already adjacent to to gain combat advantage against, or shift 0 squares when you're already adjacent to an enemy?