Developer Mike Swanson has created a new utility for icon creators:

Early on, I noticed that the rounded rectangles I was making in Photoshop didn’t match the screenshots of my app running on iOS 7. Further investigation revealed that the underlying functions that create rounded rectangles in iOS 7 were all-of-the-sudden creating these new, smoother corners. […]

The script includes presets for common iOS icon sizes, but all values and options can be modified.