3 April 2013John Gruber, via Branch:
What I’ve heard: iOS 7 is running behind, and engineers have been pulled from OS X 10.9 to work on it.
Word on the street is that iOS engineers with carry privileges all have some sort of polarizing filter on their iPhone displays, such that it greatly decreases viewing angles, thus making it difficult for observers to see the apparently rather significant system-wide UI overhaul.
iOS 7 has to be ready for the new iPhone. 10.9 doesn’t have the same scheduling constraint.