there is no standard that says it has to be in a certain order.
See this for a related question: How do you stop Chrome and Opera sorting JSON objects by Index ASC?
note: we're talking about a PHP function, but the result is basically javascript, so the statement about the non-existing standard applies as well.
btw: I have tested it with the following code. PHP itself doesnt seem to sort the array, firefox doesn't as well (according to the firebug console).
<pre>
<?php
$array = array();
$array[289] = array('name'=>'One');
$array[292] = array('name'=>'One');
$array[290] = array('name'=>'One');
print_r($array);
$string = json_encode($array);
print_r($string);
?>
</pre>
<script>
var foo = <?=$string?>;
console.log(foo);
</script>