It's been moved into one of the add-on packages for Xcode, and is now part of the Graphics Tools for Xcode package.
Apparently this is (at least partly?) because this is no longer the 'officially recommended' way to generate (high resolution) icons. See the OS X Human Interface Guidelines and the High Resolution Guidelines for more info.
I believe Icon Composer doesn't support the highest resolution icons required these days (1024x1024), and you're encouraged to use iconutil instead, which does, and allows you to convert between ICNS and "iconsets" (effectively just a folder containing a collection of PNG files of different resolutions).
If you still want it, you can get it as follows:
In Xcode, go to the Xcode menu, then> Open Developer Tool and select> More Developer tools....
You'll need to log in (or register) with your (free) Apple Developer account, and then you'll be shown a list of available packages for Xcode, including the Graphics Tools for Xcode. Grab the latest one and install.
This DMG of Graphics Tools contains:
- CI Filter Browser
- Icon Composer
- OpenGL Driver Monitor
- OpenGL Profiler
- OpenGL Shader Builder
- Pixie
- Quartz Composer
- Quartz Composer Visualizer
- Quartz Debug
