Starting later this week, we will no longer be automatically
populating the in_reply_to_status_id attribute of statuses. If your
application wants to specify that a status is in reply to a specific
status ID, please include the in_reply_to_status_id parameter when
POSTing to /statuses/update.
We will continue to populate the in_reply_to_user_id attribute based
on the screen_name at the the beginning of an @reply-style tweet. This
means that even if you don't specify the particular status ID you're
replying to, you can still ensure that a status will show up in
another user's /statuses/replies feed (and the "Replies" tab on the
Twitter website).
This is all a bit confusing, but it boils down to this:
1. If you're replying to a particular tweet, make sure to include that
tweet's ID in the in_reply_to_status_id parameter in addition to
ensuring that "@username" begins the body of the update.
2. If you just want to post a public tweet to another user, start
tweets with "@username" like you always have.
Hopefully, a very minor change. Let us know if you have any questions.
--
Alex Payne - API Lead, Twitter, Inc.
http://twitter.com/al3x