Newer Versions
As of Xcode 4.4, Icon Composer is no longer bundled with the IDE. However, you can still download the program from Apple's developer downloads page (developer.apple.com/downloads/)
Search for "Graphics Tools" and download the latest version of the tools, which is currently the version for Xcode 5.1
Older Versions
In Xcode 4.3.1 and up Icon Composer can be located in this directory.
/Applications/Xcode.app/Contents/Applications/
In your applications folder right click on Xcode and click "Show Package Contents."
EDIT: to make this answer as complete as possible.
However, In Xcode versions 4.2 and below, Icon composer can still be located in the following directory.
/Developer/Applications/Utilities
And of course, as Dave DeLong said below, Icon Composer can also be accessed through Xcode by navigating to Xcode Menu >> Open Developer Tool >> Icon Composer.