Jim, suggested change on latest version: documentation says that if $publickey is missing, it will auto disable. This is not quite so. It will not appear on the form pages but the check will still be done and will fail. To complete this feature, change

classes/observers/class.google_recaptcha.php

in the constructor right before the attach to do:

if (empty($publickey)) $pages_to_check = array();

Tested and verified in 1.5.6c.