If you need root view of your activity (so you can add your contents there) use
please note that as Booger reported, this may be behind navigation bar (with back button etc.) on some devices (but it seems on most devices it is not).
If you need to get view that you added to your activity using
setContentView() method then as pottedmeat wrote you can use
final ViewGroup viewGroup = (ViewGroup) ((ViewGroup) this .findViewById(android.R.id.content)).getChildAt(0);
But better just set id to this view in your xml layout and use this id instead.