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Yes, it is natural to me too. If I am at the home screen and click on an email notification, I expect to read the e-mail, hit back and be back on the home screen, not in the inbox.

Thanks for the explanation!

(Yeah, I realize that using the back button to invoke the application's back function would cause its own set of issues. It's a case where the same button needs to do two different things based on the user's current mindset, which the phone obviously won't know about.)

Honeycomb adds the equivalent of the "up" button you're describing here. The convention is that in the upper left corner of a screen there's a button that takes you to your "home within the app" (e.g. the inbox for Gmail). I expect that this will filter into phone UIs with Ice Cream Sandwich.