Twitter announced some changes that make it harder for third-party apps to access your account, and thus produce a fluid Twitter experience for end users.
In essence, Twitter is switching from something called xAuth to oAuth authentication. As Daring Fireball's John Gruber goes into further detail on, all third-party apps now need to use oAuth for user authentication, which forces them to exit the app and go into a browser window for authorization on Twitter.com. Compare that to the seamless login flow you now enjoy on third-party apps where all you need to do is input your user name and password in the app.
But, as Gruber points out, Twitter's own app will not need to go through the hassle of oAuth web authorization.