Reminds me a lot between the G1 and G3 curvature functions from Autodesk Alias (http://www.youtube.com/watch?v=GSIIU03NGlY). More control points are used for more curvature continuity (see also http://en.wikipedia.org/wiki/Freeform_surface_modelling).

This is probably the best analysis that I've seen of the iOS 7 icon shape: http://blog.mikeswanson.com/post/62341902567/unleashing-gene...

(I am slightly biased, though.)

That's much better than the "superellipse" formula that was going around a few weeks ago. It was a good try, but it was pretty imprecise (based on an image that was too low-res to accurately compare).