Android Centering the content

Android Centering the content

I'm a total beginner in Android layout, but I want to center the content
vertically and horizontally.
So this book I'm reading says this:
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:background="@color/background"
android:layout_height="match_parent"
android:layout_width="match_parent"
android:padding="30dip"
android:orientation="horizontal" >
<LinearLayout
android:orientation="vertical"
android:layout_height="wrap_content"
android:layout_width="fill_parent"
android:layout_gravity="center" >
...component.. etc...
This works and I think i get it, except Eclipse says:
This linearlayout or its parent is useless; transfer the background
attribute to the other view
But I do not understand this?