In my opinion - the turning point of Part I isn't the meeting with Sollazzo or Michael's time in Sicily - although they both are extremely important - its the time Michael is in the hospital visiting his father, and takes control of the situation in moving his father to a different room to save his life.
On a seperate thread someone said Michael whispers to Vito, "I'm with you now" or something like that. I have never heard that in the movie - Michael does whisper something but i cannot hear it- but that would not surprise me.
What happens from here on out starts with that action.
