Hi
In fact, as far as I know there are no validations (in terms of some approvals) for PM orders.
Try to look on user statuses functionality. In the past I was using it to get some additional levels of approvals before release of order.
http://help.sap.com/saphelp_rc10/helpdata/en/cc/b25828844811d2a5be0060087a7a74/content.htm
Heniek