Twitter has put up a blog post ostensibly to inform users that, in order to better secure and keep our direct messages (DMs) private, 3rd party apps will now be forced to use the web-based OAuth login method rather than the more native-feeling xAuth most of the use today.
When you first connect an application to Twitter, we’ll give you more detailed information about what you’re allowing the app to do with your account. These activities may include reading your Tweets, seeing who you follow, updating your profile, posting Tweets on your behalf, or accessing your direct messages. If you’re not comfortable with the level of access an application requests, simply say “No, thanks”.
So what this boils down to is, when you get a new Twitter app -- anything from a full client like Twitterrific or TweetBot down to apps that simply allow you to share content via Twitter, such as games and Instagram -- you won't be able to simply enter your username and password in the fields like you do today. Instead the app will have to call up an embedded browser window (UIWebView) and load up a Twitter.com page which then presents the more complex screen shown above.