Very nice. There is one bug: The icon size between 128 and 32 is 48, not 64; that's why the icon is clipped in the 64-pt Carbon image well (it uses the next larger rep, which is 128, and you told it not to scale, so it clips).

Also, I wrote a similar app called IconGrabber. I was still learning Cocoa at the time, and hadn't yet found out that iconForFileType: could do HFS types; it uses GetIconRef for type/creator combinations. You can supply a creator, or leave it blank to use kSystemIconsCreator. It also supports NSImage names, filename extensions (iconForFileType: again), and MIME types, as well as various icon transformations (e.g. disabled, selected, scaling). And you can save any icon as a TIFF file.

Mine doesn't have any reusable categories, though, so yours has the advantage there. :)