I am trying to assign relative widths to columns in a ListView that is in a TabHost, using layout_weight as suggested here:

<?xml version="1.0" encoding="utf-8"?>
<TabHost xmlns:android="http://schemas.android.com/apk/res/android"
  android:id="@android:id/tabhost"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent">
  <LinearLayout
     android:orientation="vertical"
     android:layout_width="fill_parent"
     android:layout_height="fill_parent">
     <TabWidget android:id="@android:id/tabs"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"/>
        <FrameLayout android:id="@android:id/tabcontent"
           android:layout_width="fill_parent"
           android:layout_height="fill_parent">
           <TableLayout  
              android:id="@+id/triplist" 
              android:layout_width="fill_parent"
              android:layout_height="wrap_content"
              android:paddingTop="4px">
              <TableRow>
                 <ListView android:id="@+id/triplistview"
                    android:layout_width="fill_parent"
                    android:layout_height="wrap_content"/>
              </TableRow>
              <TableRow>
                 <Button android:id="@+id/newtripbutton"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="Add Trip"/>
              </TableRow>
[other tabs ...]

My row definition has 4 columns that I would like to size as follows:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   android:layout_width="fill_parent"
   android:layout_height="wrap_content"
   android:orientation="horizontal"
   android:weightSum="1.0"
   android:padding="4px">   
   <TextView android:id="@+id/rowtripdate"
      android:layout_weight=".2"
      android:layout_width="0dip"
      android:layout_height="wrap_content"
      android:inputType="date"/>
   <TextView android:id="@+id/rowodostart"
      android:layout_weight=".2"
      android:layout_width="0dip"
      android:layout_height="wrap_content"/>
   <TextView android:id="@+id/rowodoend"
      android:layout_weight=".2"
      android:layout_width="0dip"
      android:layout_height="wrap_content"/>
   <TextView android:id="@+id/rowcomment"
      android:layout_weight=".4"
      android:layout_width="0dip"
      android:layout_height="wrap_content">

Unfortunately, it seems to want to fit all the columns into the space that the button occupies, as opposed to the width of the screen. Or maybe there is another constraint that I do not understand. I'd appreciate your help.