As written (ie before the ECJ's judgements are superimposed over the top of text of the badly written Regulation) the Regulation only talks about delays in departure, not arrival.
The ECJ says that the Regulation should also apply to delays of over three hours, but has not included an interpretation if this is on departure or arrival.
So a flight with an on time departure which diverts isn't covered by the Regulation (One could argue). Can hardly wait for the lawyers to argue this one.