AndroidUpTheme

<code>
<resources>
    <style name="Theme.AppTheme" parent="android:Theme.Holo">
        <item name="android:homeAsUpIndicator">@android:color/transparent</item>
    </style>
</resources>
</code>

And apply it to the application, (Be aware of values-v14 etc)

<application
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/Theme.AppTheme" >

If your using sherlock youll need to double up the xml values as discussed on their website.
Its been tested and works. Let us know if you need any more help.