It seems that Apple guidelines as of August 3, 2010 now include the “High resolution” images (for iPhone 4) in their “required” icon sizes.
Looks like we need to provide both a 57×57 and a 114×114 image now, as well as a 640×960 title image.
See Custom Icon and Image Creation Guidelines (Javascript required) which is part of a whole document:
- iOS Human Interface Guidelines (2013; by Apple Inc; PDF; 26,3 MB)