EDIT Until such time as StackOverflow allows us to version our answers, this is an answer that works for Android 3 and below. Please don't downvote it because it's not working for you now, because it definitely works with older Android versions.
You should only need to add one line to your onCreateDialog() method:
@Override
protected Dialog onCreateDialog(int id) {
//all other dialog stuff (which dialog to display)
//this line is what you need:
dialog.getWindow().setFlags(LayoutParams.FLAG_FULLSCREEN, LayoutParams.FLAG_FULLSCREEN);
return dialog;
}