I think the updated design guide for ICS Android calls for the back button to ascend up the hierarchy of the app, rather than taking you to whatever screen you were last on. (So it is now supposed to be deterministic.) There are also better guidelines for how to provide in app navigation.
The new navigation guidelines have back behaving as previously, but call for an "Up" button that navigates the app's hierarchy.
http://developer.android.com/design/patterns/navigation.html
-wrong below!-
I think the updated design guide for ICS Android calls for the back button to ascend up the hierarchy of the app, rather than taking you to whatever screen you were last on. (So it is now supposed to be deterministic.) There are also better guidelines for how to provide in app navigation.
It'll probably take quite a while to catch on.