I couldn't find anything online, so while trying out the newly released Sketch 2 I put together this simple template file for producing and exporting iOS icon files. It's not complete – there's no settings icon for example – but it includes templates and slices for the iPhone and iPad and the large App Store icon.

iOS Icon Template for Sketch 2

I make no guarantees about its accuracy or usefulness in general. Icon pixel sizes and corner radii were provided by Jon Hick's handy icon reference chart.