In Mac OS X 10.6, Apple introduced support for PNG-compressed icon data in .icns files. In Mac OS X 10.8, Apple introduced support for Retina icons and the iconutil command-line tool for generating icons from a folder of PNG files.
However, iconutil only knows how to generate PNG-compressed icon data, which means that you can't use it to create a Retina icon that will still display properly on Mac OS X 10.5.
He’s written some code that converts .icns files to use JPEG 2000 internally instead of PNG, which makes them readable on Mac OS X 10.5. Kudos to Kusterer for figuring this out, and shame on Apple for silently breaking compatibility.