This has been driving me nuts for a while now.
Is there any way of reliably detecting if the home button has been pressed in an android application?
Failing
Since you only wish for the root activity to be reshown when the app is launched, maybe you can get this behavior by changing launch modes, etc. in the manifest?
For instance, have you tried applying the android:clearTaskOnLaunch="true" attribute to your launch activity, perhaps in tandem with android:launchMode="singleInstance"?
Tasks and Back Stack is a great resource for fine-tuning this sort of behavior.