... and in your strategy you can list what you know to be out of scope, then have it reviewed/signed off by those with a stake in your test efforts. This will encourage team participation in your efforts.
Deciding the scope is most important part of Test Strategy and Planning.
Like Jake mentioned, Document what you think is in scope and what is out of scope and get it signed off by all stake holders in the Project.
You may not get it right for the first time and may have to have multiple versions of the doc, but finally once it is signed off there can no blame game.
So, the theory is: Assume and confirm.