Looks like a good resource:

When we released our previous file in 2009, the main question we received was why we would go to the trouble to create an Illustrator version of the iPhone UI elements. We addressed this at the time in a post titled “Why build iPhone app mockups in vector format?” There is even a nice note about the then rumored imminent introduction of higher resolution screens and how we hoped our use of vector files would help us once they were announced. (It helped A LOT.) […]

[W]e’ve purposely turned “Align New Objects to Pixel Grid” off in the Transform palette because it wreaks havoc on corner radii and icons. We recommend that you do the same.