Zend_Json::encode checks for and uses the method "toJson" on the passed object (if its an object and the method exists). This is a very handy trick. However, when used from the Json view helper, the Zend_Json_Encoder::_encodeValue() method does not make the same check.

Here is my fix (added first two lines to function):

    protected function _encodeValue(&$value)
    {
        if (is_object($value) && method_exists($value, 'toJson')) {
            return $value->toJson();
        } else if (is_object($value)) {
            return $this->_encodeObject($value);
        } else if (is_array($value)) {
            return $this->_encodeArray($value);
        }

        return $this->_encodeDatum($value);
    }