I think this is what you are looking for.

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent" android:layout_height="fill_parent">

    <Button android:layout_width="fill_parent"
        android:layout_height="wrap_content" android:id="@+id/testbutton"
        android:text="@string/hello" android:layout_alignParentBottom="true" />

    <ListView android:layout_width="fill_parent"
        android:layout_height="fill_parent" android:id="@+id/list"
        android:layout_alignParentTop="true" android:layout_above="@id/testbutton" />

</RelativeLayout>