you're right, I just installed TweetPhone+ and it allowed me to see conversations of a tweet that I wasn't part of. However, I am pretty sure that is because that app uses an older version of Twitter API (it hasn't been updated in a long time). I say so because Mehdo dev had mentioned that this feature was removed from Twitter API v1.1, which is probably what all the recently updated Twitter apps use. I'll confirm with Liquid Daffodil and Mehdoh's dev over Twitter.
Update: Apparently, this was never part of Twitter APIs (MeTweets dev confirmed). And apparently, TweetPhone+ is using some kind of workaround to get this working. The good news, though, is that the devs of Gleek, MeTweets and Mehdoh know about this (TweetPhone+getting it to work) and will definitely come up with a solution, if one exists....or hopefully a work-around at least.