I think this in an interesting discussion. Sometimes the user presses back to go to the previous screen, sometimes to go to the parent element (i.e., when you're seeing an email and press back to return to your inbox).

For me, the best solution would be make the back button come back to the previous screen, ignoring where they are. So, if you are playing Angry Birds, tap an email notification and then press back, you return to Angry Birds. But, also, in screens in which the user may want to go to the parent element, developers should put a button to go "home". In the email screen, a button to go back to inbox, in the Music app a button to go to the Music app initial screen...