ActionBarSherlock translúcida

En la actividad

@Override
protected void onCreate(Bundle savedInstanceState) {
    requestWindowFeature(Window.FEATURE_ACTION_BAR_OVERLAY);
    getSupportActionBar().
        setBackgroundDrawable(
            getResources().getDrawable(R.drawable.actionbar_bg)
        );
    // ...

res/drawable/actionbar_bg.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
android:shape="rectangle" >

<solid android:color="#64000000" />

</shape>