When in_reply_to_status_id was originally added you could "reply" to a tweet without including the @username in the tweet, and twitter would accept that (and thread that) as a proper reply. On the one hand this freed up a few additional characters for the reply, but also lead to confusion since people were seeing replies which didn't include their screen_name.
This behavior was later changed to require that the screen_name which authored the tweet being replied to be mentioned in the reply (this was around the time that replies became "mentions"). A tweet posted with in_reply_to_status_id but without the related screen_name would get posted, but the in_reply_to_status_id field would not get set. -- -ed costello -- Twitter developer documentation and resources: http://dev.twitter.com/doc API updates via Twitter: http://twitter.com/twitterapi Issues/Enhancements Tracker: http://code.google.com/p/twitter-api/issues/list Change your membership to this group: http://groups.google.com/group/twitter-development-talk
