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…
It has unfortunately little to do with UX, since both end up being confusing on edge cases (Astro's horribly enraging "press back twice to 'exit'", the conversation or gmail app issues people mentioned in this thread).
I personally tend to prefer back as in back in time but I know for sure opinions vary.