[jsb]

So long as you start the tweet with @[twitteruser], your @reply will only appear in someone's feed if they follow you and also the person you are sending the at-reply to. Or if they are the person you're sending the message to, of course.

[vaksel]

that's the point, if my message is only to that one person, why would my followers want to read about it?

[jsb]

To clarify, your @reply won't necessarily appear in everyone's feed. It will only appear if:

1) They are the user you are sending the @reply to, OR 2) They follow you AND the person you send the @reply to.

So for example, if I follow @balsamiq and @fogbugz and @balsamiq sends an @reply to @fogbugz, that message will appear in my feed. But if @balsamiq sends an @reply to a user I do not follow, I will not see it in my feed.

[davidu]

I'm pretty sure it doesn't work that way. I think everyone see's your @reply -- perhaps you mean DMs?

[icey]

jsb is correct; they made this change a few months back.

[shaddi]

Extra tidbit: this is the reason for those ". @foo ..." tweets you may be seeing. If any character precedes the @, then the tweet will appear in all your follower's feeds; otherwise, it behave as jsb described.