I'd think that the "typical" behavior would be to go back to what you were doing. I'm playing "Angry Birds", notification shows for text message. I click it, read the notification, hit back to get back to Angry Birds.

I don't see how inserting items between the text message and Angry Birds is a good idea.

What apps should have, is a way to always get to the apps main screen in the app. But changing the back button behavior seems confusing.