Generally, if I come to an activity from another app, it's to do one specific thing and then I want to go back to the original app. There are certainly cases when I want to stay in the app I got sent to and then hit back and find myself in the original app. The path then to the app I want is the same as always: home button, app shortcut.

Conversely, on iOS, what happens when I get sent to a different app and then want to go back to the original app? Home button, app shortcut. In my experience, the android way addresses the more common scenario.

Also, there are builds of android (HTC's Sense UI, I believe) that do have notifications on the lock screen. Not only that, but the lock screen is configurable, so you can add shortcuts that, when you drag them to the unlock field, take you directly to the app. This is a neat feature that I hope eventually makes it into android proper.

EDIT: Hm, this is factual and fairly uncontroversial. I suppose getting downvoted sometimes is just an inevitable consequence of participating in any dicussion involving iOS and Android? Oh well.

But you're right, there isn't a good way to go to an app's home screen if you enter from a specific activity. That's something that needs to be looked at, but not at the expense of changing the excellent back button functionality.