The "retweeted status" is only available if the tweet is a "retweet". Thats why I was suggesting there be a variable like "retweeted_by_me" in the original tweet to let you know you've retweeted it so you can Undo.
If this were to be implemented there needs to be a new method to Undo the retweet which takes in the original tweet id. Since you don't have the id of the retweet you cannot call statuses/destroy. If you call it using the original id it results in an error since you don't own the tweet. Right now retweets are a one-way action with the API. If you need to unfavorite a retweet you must go to your favorites and unfavorite the tweet and if you want to Undo a retweet you need to use statuses/ retweeted_by_me and destroy the retweet. It seems like twitter.com handles favoriting and retweeting well so there seems to be some discrepancy between what twitter.com uses and the API.
