Benefit: When a nonadjacent enemy enters a square adjacent to you, you can make an opportunity attack with a polearm against that enemy, but you grant combat advantage to that enemy until the end of the enemy’s turn.[PHB:204]
Notes: According to WOTC, due to the wording of the feat, entering the square includes shifting. However, it still does not include forced movement such as a push, pull or slide. It must be "active" movement.
Notes2: The power was fixed by the December 2009 update. Since then it doesn´t work in shifting.
Notes3: The feat still functions normally and is triggered by shifting. Shifting cannot provoke opportunity attacks. However, in this case, shifting isn't provoking an opportunity attack, but instead allows the user to make one.
No word on whether teleporting into the square triggers the feat.
Also, as a reminder (to address a commonly asked question with this feat), you cannot take opportunity attacks during your turn, even if an enemy moves during your turn.