$view = new Zend_View();
$view->foo[] = 42;
var_dump($view->foo);

The output is NULL.

Reason: If you note $view->foo[] PHP calls the __get() method. The result of isset($this->_vars[$key]) is false so null is returned. The returned null is not assigned to $this->_vars['foo'] so the "= 42" results into an assignment of 42 to a formerly null somewhere in the memory and gets lost.

2nd example:

$view = new Zend_View();
$view->foo = array(); // initialising of foo
$view->foo[] = 42;
var_dump($view->foo);

The output is array(0) {}.

Reason: $view->foo = array(); calls __set() and an empty array is assigned to $this->_vars['foo'].
$view->foo[] calls __get() but returns a copy of $this->_vars['foo'] because it is not returned by reference. The assingment "= 42" works on the copy and gets lost.

My suggestion for Zend_View_Abstract::__get() is:

public function &__get($key)
{
    /**
     * @todo exception?
     */
    if ($key[0] == '_') {  // [1]
        $null = null;
        return $null;
    }
    if (!isset($this->_vars[$key])) {  // [2]
        $this->_vars[$key] = null;
    }
    return $this->_vars[$key];
}

This is the exception-free version.
[1] This part is a bit ugly. I'd prefer to throw an exception instead.
[2] Every read access to an unassigned property creates a null-entry in $this->_vars[] but I don't think that this is a mission critical behaviour.

Btw:

Zend_View_Abstract::__isset() should be changed to use isset($this->_vars[$key]) instead of array_key_exists($key, $this->_vars) because the normal behaviour of isset() on null values is to return false. Current behaviour:

$foo = null;
var_dump(isset($foo)); // output: bool(false)

$view->foo = null;
var_dump(isset($view->foo));  // output: bool(true)