I\'ve inherited the Holo Light Theme and customized the background of the ActionBar with the following:
Content of styles.xml
Ok, thanks to Romain Guy on #android-dev IRC channel, it's a known bug on honeycomb / Android 3.0 which will be fixed on the next release. Since then, the only solution is do it from code, and it works :-)
final ActionBar actionBar = getActionBar();
BitmapDrawable background = new BitmapDrawable (BitmapFactory.decodeResource(getResources(), R.raw.actionbar_background));
background.setTileModeX(android.graphics.Shader.TileMode.REPEAT);
actionBar.setBackgroundDrawable(background);