Drush doesn't function without Console_table. This means that if drush is installed and then executed by a user, it is useless, as it will just fail with "Drush needs a copy of the PEAR Console_Table library in order to [error] function, and the attempt to download this file automatically failed because you do not have permission to write files in /usr/share/drush/includes. To continue you will need to download the 1.1.3 package from http://pear.php.net/package/Console_Table, extract it, and copy the Table.php file into Drush's directory as /usr/share/drush/includes/table.inc." And if executed as root, then drush succeeds in automatically downloading the library, resulting in files untracked by Portage in the system. The dep on pear should be replaced by dev-php/PEAR-Console_Table.
Pushed fix to drush-4.5-r1 and drush-5.1. Thanks for reporting!