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TechCrunch first reported the tip, citing multiple unnamed sources, Monday afternoon. And now All Things D seems to have confirmed the story as well, going so far as to say the service will be announced this week. ATD's story also cites unnamed sources but claims the announcement will be made at its conference, D9, this week in Rancho Palos Verdes, California.
It wouldn't come as too huge of a surprise if these reports turn out to be true. After Twitter asked developers to stop building third-party clients, the company announced its acquisition of TweetDeck. It did take months for those rumors to finally be confirmed, however, so we may be hearing these rumors for some time before Twitter makes any official announcement.