I've just had a user who's search toolbar button in Outlook 2003 (no SP) has disappeared. Also the EV menu item under Tools has gone. Apart from this archived items are still shown with the correct icon and can be retrieved.
Initially when the user was first enabled, this was working.
I've tried clearing the forms cache (just for good measure) but no joy. The user is also using offline folders.
I think the disable PST setting prevents users loading the Personal Folders option in Services on the Tools menu for Outlook 2000 or the hits the registry setting for Outlook 2003 to stop the item appearing on the File menu. I do not believe it will affect.OST as this is loaded by Outlook itself.
You are right though, an .OST is just a glorified .PST...