[Based on ZF 1.5.2 code]

Such call => Zend_View_Helper_PartialLoop::partialLoop($viewScript, Zend_Db_Table_Rowset $rowset) will couse this:
---------
    public function partialLoop($name = null, $module = null, $model = null)
    { 
	// ...
	// ...	
        if (is_object($model) && method_exists($model, 'toArray')) {
            $model = $model->toArray();     
        }
--------

to be always true, because Zend_Db_Table_Rowset which is stored in $model here implements toArray() method, whoch does this:

--------
    public function toArray()  
    { 
        // @todo This works only if we have iterated through
        // the result set once to instantiate the rows.
        foreach ($this->_rows as $i => $row) {
            $this->_data[$i] = $row->toArray();
        }
        return $this->_data;   
    } 

--------
So all rows in rowset are converted to array.


The side effect of this behaviour is that:
$this->partialLoop()->setObjectKey('myObject')->partialLoop($viewScript, Zend_Db_Table_Rowset $rowset);
won't populate row object inside $this->myObject, which behaviour is in fact implemented in Zend_View_Helper_Partial::partial():
--------
        if (!empty($model)) {
            if (is_array($model)) {
                $view->assign($model);
            } elseif (is_object($model)) {
                if (null !== ($objectKey = $this->getObjectKey())) {
                    $view->assign($objectKey, $model);              // <== This line is never reached, despite we have called setObjectKey('myObject')
                } elseif (method_exists($model, 'toArray')) {
                    $view->assign($model->toArray());
                } else {
                    $view->assign(get_object_vars($model));
                }
            }
        }

--------


Commentig out line 71 in lib/Zend/View/Helper/PartialLoop.php, it is "$model = $model->toArray();" makes object accessible insiade partial view via $this->myObject.