Here's the official answer:
Changing your password on DIRECTV.com will not invalidate your saved token inside the iPad application.
Only a logout, or a deletion of the app will.
Sounds pretty definitive!
Mr. young: I am pretty sure there are ways to remotely disable your iPad should it become lost or stolen. I should look into it, as I just got a new one. You can certainly enable "Find my iPad", which I am about to do!