This is one for which I’ve always wondered why guys are so intent on “staying out of the end zone.” Unless I’m mistaken my interpretation of the rule (in college) is that the ball is dead/down at the spot a player achieves control of the ball, even if his momentum carries him into the end zone. As long as he “possesses” the ball prior to the ball breaking the plane, the ball is down at that spot and is not a touchback.
Edit: I’ll amend my statement by clarifying that all of this applies to a kicking team player. If the kick returner catches the ball in the playing field and goes into the end zone, the ball is still live and the returner must bring the ball back outside the end zone. If he doesn’t bring it back out of the end zone, I believe that results in a “safety.”