It'd be great if .@ would still count as a reply (i.e. that tweet would still contain a pointer to the tweet it's replying to) but be visible by everyone. It's the best of both worlds.
Also I wonder what happens if you hit Reply, but then delete or modify the @username. Is it still counted as a reply to that person's tweet, and therefore only visible to the intersection of your followers?
* Having the @username anywhere in the tweet preserves the pointer.
* Deleting the username clears out the pointer.
* Deleting the username, and retyping it (at the end of the tweet) restores the pointer.
The last scenario makes me think that the validation is server side. It seems that Twitter ensures that the tweet still mentions the author of the replied-to tweet or it will clear out the pointer.
Replies are triggered by two factors. An `in_reply_to_status_id` field that is filled in behind the scenes and an @mention of the original author in the text.
I think that hitting "reply" should still contain a pointer even if you don't include the @username. Doesn't the threaded-reply already imply that you're addressing that person?
I think that would require a tweak to the UI to make it more obvious that a tweet is a reply. There's already some subtle stuff there (small-font "in reply to" and I think a little curved arrow), but the most obvious part that the content starts with "@".
Also I wonder what happens if you hit Reply, but then delete or modify the @username. Is it still counted as a reply to that person's tweet, and therefore only visible to the intersection of your followers?
EDIT: s/union/intersection