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.
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 errata. Since then it doesn´t work in shifting.
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.