Just for anyone who might have the same problem I did be aware that you cannot set the visibility of action methods as protected in ZF 1.5, as you could in 1.0.x - you must set their visibility to public, otherwise a Zend_Controller_Action_Exception error will be thrown similar to:

Action "index" does not exist and was not trapped in __call()

I did not find this documented in the changelog or elsewhere and caused some confusion before finally resolving the issue.