The PHP version is not correctly detected
Drush doesn't use just php directly as a command, but need the full path. With the last version of Drush, you can add the following line in your .bash_profile to set which PHP you want to use.

export DRUSH_PHP=/Applications/MAMP/bin/php/php5.3.14/bin/php

Here is just an example of MAMP, but you can change it as well with your own path.