NOTE: #1064472 is the big change here. It separates the old field export into two new exports "field_base" and "field_instance". This helps distribution developers export base field definitions and allows users to change field instances without causing the distribution feature to be marked as overridden.
Version 6.x-1.2 and 7.x-1.x-beta5 (or later) will no longer work with Drush 4.4 or lower. In order to support the upcoming Drush 5 (and continue to support Drush 4.5) we needed to break back support for older versions of Drush.