Miso Antonic wrote to tell me about a post on the Paintcode blog:
In iOS 7, Apple introduced a slightly different icon shape – it is no longer a simple rectangle with circular arcs in corners. The old design produced a subtle but noticeable “hump” in places where arc touches some of the straight lines. The new iOS 7 icon shape looks more “organic” and eliminates this effect.
We’ve decided to open source some Objective-C categories for drawing the new iOS 7 rounded rectangles.
Included in the post is a good summary (including visuals) of the changes in iOS 7 rectangle rounding.