The closest thing I’ve seen to this is the Do Not Disturb feature while driving on the iPhone. You can set it to allow for select contacts (your favorites) to get through to you if they call twice in a row- which would indicate, presumably, that there is an emergency.

It is crazy, though, that this notification level situation isn’t really mainstream. I can see applications for it in email (although they have the ! for “urgent” emails which everyone just ignores at this point) as well as phones.