Thank God. To be super-optimistic, how great would a rotation be where Pettitte's your #3 starter behind Johan Santana and Chien-Ming Wang?
The last time Pettitte was a true #3 was 2005, and Houston made the World Series. He was, off the top of my head, also probably a #3 in 1998 (David Cone and Wells) and 2001 and '03 (Roger Clemens and Mike Mussina). The Yanks made the World Series in all three years.
Does it mean much? Probably not. But in my mind, Pettitte's return is the most unadulterated positive news this off-season for the Yankees.

215 more overrated innings! In all seriousness this is big for the Yankees. If they land Santana the have the best top three in baseball. Figuring one of Kennedy/Joba/Mussina have a decent season this is as good of a rotation they've had in years. On the other hand I still believe Santana won't be traded and will make it to free agency. That said, it is key that Pettitte is back because of the youth on the staff. Either way, his presence makes them better.