*
 * ERROR: app-arch/unzip-5.52-r1 failed.
 * Call stack:
 *               ebuild.sh, line 1641:  Called die
 * The specific snippet of code:
 *      preprocess_ebuild_env || \
 *              die "error processing environment"
 *  The die message:
 *   error processing environment
 *
 * If you need support, post the topmost build error, and the call stack if relevant.
 * A complete build log is located at '/var/log/portage/app-arch:unzip-5.52-r1:20080329-123442.log'.
 * The ebuild environment file is located at '/var/tmp/binpkgs/app-arch/unzip-5.52-r1/temp/environment'.
 *
 * The 'prerm' phase of the 'app-arch/unzip-5.52-r1' package has failed
 * with exit value 1. The problem occurred while executing the ebuild
 * located at '/var/db/pkg/app-arch/unzip-5.52-r1/unzip-5.52-r1.ebuild'. If
 * necessary, manually remove the ebuild in order to skip the execution of

***************************************************************************

This seems to suggest unzip is already installed:

emerge -S unzip
Searching...
[ Results for search key : unzip ]
[ Applications found : 2 ]

*  app-arch/unzip
      Latest version available: 5.52-r2
      Latest version installed: 5.52-r2
      Size of files: 1,113 kB
      Homepage:      http://www.info-zip.org/
      Description:   unzipper for pkzip-compressed files
      License:       Info-ZIP