Using layout_gravity="bottom" to place at bottom of LinearLayout

I would like to place a layout on the bottom of a LinearLayout, but I can't seem to get it to work. I know that I can use RelativeLayout to do this, but I should be able to use LinearLayout, shouldn't I?

EDIT: Actually this is more confusing than I thought. The layout below is simplified. In reality, I'm using fragments on a pre-3.0 device, with the compatibility layer.

I used Hierarchy Viewer to examine what's going on, and found that an android.support.v4.app.NoSaveStateFrameLayout was added to my layout, and that layout has layout_height set to wrap_content. That seems to be what's causing my problem, but I haven't yet figured out how to fix it.

Specifically, why doesn't this work? Shouldn't the layout_gravity place it at the bottom?

<LinearLayout
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">

        ... stuff here ...

        <LinearLayout 
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:layout_gravity="bottom"
            android:orientation="horizontal">

            ... more stuff here ...

     </LinearLayout>
</LinearLayout>

BTW, changing layout_height to fill_parent or setting layout_weight don't seem to work either. I just want to better understand what is going on, because clearly I'm missing something important. Thanks.