The head honcho in charge at Polyphony digital and the development of GT5, Kazunori Yamauchi, is one of the industries biggest perfectionists. He'll not release GT5 until he's satisfied that everything that can be done to make the game better, has been done.
How would you feel if they released the game in the summer, and it was a dissapointment due to overhyping. ?
I can confidently say, that when we do eventually see the final build of GT5, it will decimate the competition and will raise the bar to a whole new level, as it has done consistently over the years.
Think of this delay as nothing more than a great game, being made even greater.