Already made this point, but just want to expand on it a little bit. The officials really killed the Knicks down the stretch. First they get Melo on the push off, after PP had his arm draped all over Melo's shoulder. Then KG sets an illegal screen (trips Douglas), which helps free up Allen for the game winner.
http://yfrog.com/h0t0cvyj
I don't want to sound like I'm blaming the entire game on the officials, because, at least to some extent, the Celtics executed better than the Knicks did down the stretch. But poor officiating on two of the final three possessions was one of the deciding factors.
EDIT: And we're talking about a flat-out dirty play by KG. Blatantly trips Toney Douglas. No other reason to stick the leg out like that. But final possession for the home team in a NBA playoff game ... no whistle.
http://www.celticstown.com/2011/04/17/video-ray-allen-game-winner-against-knicks/