This one is a little embarrassing.
Just trying to install Magento on a new (fresh) container. Running across the following style of error (when executing ./pear install magento-core/Mage_All_Latest-stable) :

Starting to download Lib_ZF_Locale-1.7.2.2.tgz (1,279,370 bytes)
…done: 1,279,370 bytes
install ok: channel://connect.magentocommerce.com/core/Lib_ZF_Locale-1.7.2.2
Notice: Undefined variable: php_errormsg in Installer.php on line 566
PHP Notice:  Undefined variable: php_errormsg in /home/complex/www/magento/downloader/pearlib/php/PEAR/Installer.php on line 566
ERROR: [...]