The $index argument seems to be NULL for append operations, e.g.:

<?php

// example extension class
class Test extends ArrayIterator
{
    public function offsetSet($index, $newval)
    {
        echo 'Test::offsetSet('.var_export($index, true).', '.var_export($newval, true).")\n";
    }
}

// test
$i = new Test;
$i[] = 'append a value';

?>

The result is:

Test::offsetSet(NULL, 'append a value')