onIabPurchaseFinished never called.

Try adding this to the Activity that calls mHelper.launchPurchaseFlow(..):

    @Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    Log.d(TAG, "onActivityResult(" + requestCode + "," + resultCode + "," + data);

    // Pass on the activity result to the helper for handling
    if (!mHelper.handleActivityResult(requestCode, resultCode, data)) {
        // not handled, so handle it ourselves (here's where you'd
        // perform any handling of activity results not related to in-app
        // billing...
        super.onActivityResult(requestCode, resultCode, data);
    }
    else {
        Log.d(TAG, "onActivityResult handled by IABUtil.");
    }
}

i just found out another important thing: the requestCode that is used to launch the purchase flow has to be >= 0!

i used "new Random().nextInt()" to generate a random requestCode, and sometimes it worked, sometimes it didn't. now i found out in the following documentation, that the requestCode should not be a negative number:

http://developer.android.com/reference/android/app/Activity.html#startActivityForResult%28android.content.Intent,%20int%29