So how is the message being forwarded from Outlook? Specifically is the VMO form loaded? Does the voice mail message show up in the phone interface after forwarding it from the desktop? I'm interested to see if the message class (IPM.Note.Voice.Unity) is preserved in the scenario or not - I'm assuming it is.
The notification engine _should_ get a change trigger when the message arrives and if it's a properly formed voice mail (message class and media flags determine this) it should trigger a notification update. You should be able to test this by marking a standing message read and then unread or vice versa in the outlook client and have the light go on/off (there may be a delay there, of course).