It's a pretty bold (or bald, perhaps) move from Twitter. And it's an unfortunate one because the "official" Twitter clients that I have access to (on the Web, on Windows, and on Windows Phone) are inferior to the alternatives that I use. If I'm forced to only use official Twitter clients, I'll stop using Twitter.

See http://thenextweb.com/twitter/2011/03/11/twitter-tells-developers-to-stop-developing-new-twitter-clients/ for some more about it.