One get's the impression here, that a lot of people are watching sprints only since Cav actually won some...
Peta changed directions allright? So whats wrong about that? He didn't box him near to the side and left him no way to go....
Sprinting isn't just about speed and it has never been only about it. 
Seriously that was a ****ing normal sprint. Have any of you seen Mc Ewen sprinting for wins? 
Just changing directions, even to cover, is the most normal thing you'll see in a sprint.
Not to get passed is a part of the game, the question is how you achieve it. Cav obviously hat loaaads of space to maneuver. 
Petacchi came from Cavendishs wheel and took a long sprint to the finish line, covered his ground in a way that he could be passed and in the end won. That was a completly clear, relativly clever and by the way deserved win.