Zajímavé, mě to funguje
<?php
session_start();
$_SESSION['subobj'][0]="aa";
echo $_SESSION['subobj'][0];
unset($_SESSION['subobj'][0]);
echo"<hr>";
echo $_SESSION['subobj'][0];
?>aa
Notice: Undefined offset: 0 in J:\FTP HTTP\test\test2.php on line 7