A recent post by Joel Spolsky suggests to display the reason why the menu item is not accessible by the user.
http://www.joelonsoftware.com/items/2008/07/01.html
I disagree.
The reason is, this kind of behavior would confuse the user more than disabling the menu item.
I would go with the recommended approach of hiding the menu item entirely as the user would not see the item and he would not be concerned about it anyways.
