In iOS 5 twitter integration,
TWTweetComposeViewControllerResult just has 2 options
When user selects done -- TWTweetComposeViewControllerResultDone
When user selects cancel -- TWTweetComposeViewControllerResultCancel
This result doesn't depend on the tweets updated by apple in background. If the tweets fail while updating it shows an alert.
SO i suggest do not implement any custom pop-up for success or failure. As apple itself implemented indications for success/failure tweet updates. On success it plays a sound & on failure a pop-up with reason.