This is a long overdue but very well put-together UI and Usability Guide for Android Developers. My only qualm from reading it thus far is the very last section under Navigation [1] regarding System-to-app navigation: "For the Back key, you should make navigation more predictably [sic] by inserting into the task's back stack the complete upward navigation path to the app's topmost screen." No. This piece of advice is…
Google Voice follows (or used to, at least) your advice, which led to endless frustration. You'd tap a notification to read a text, then have no way at all to get to the main screen of the Google Voice app. In Android 4.0, Google Voice has an app-specific back button that helps, but I think Google's advice on this is correct.
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.