The trick is to use layout_weight="1" and fillViewport="true" on the ScrollView.
This should do the trick:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/relativemain"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<ScrollView
android:id="@+id/scrollmain"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:fillViewport="true" >
<LinearLayout
android:layout_width="fill_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<LinearLayout
android:id="@+id/wrap"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:layout_weight="1"
android:orientation="vertical" >
//put your content here
</LinearLayout>
<LinearLayout
android:id="@+id/myadv"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:gravity="center_horizontal" >
//this is your advertisement
</LinearLayout>
</LinearLayout>
</ScrollView>
</LinearLayout>