I am glad David Cook won. The other David aka "little kid" may be a great crooner but he was severely lacking in the personality department. Cook has an incredible likeability factor and I think he will do well.
Okay, so here is my theory. The final night of competition after Cook sang his last song, remember when Simon said he thought it was the wrong choice for the final show. Cook said it was about progression and he didn't agree. Camera cuts to Simon looking at Cook - and he WINKS.
I think, that because Simon was hard on Cook, viewers felt the need to vote for Cook, because Simon's harsh words put Cook in jeopardy of losing. In fact, Simon understands how all of this works (remember the week he was nice to Carly she lost?) and the wink was letting Cook know I am with you all the way, trust me.
Further proof, Simon apologized to Cook just before the results were read, saying that when he re-watched the performances he realized it was not a knock out.