First the BlackBerry, now the iPhone! Until now, using Twitter from your iPhone required using either a third-party app or Twitter's mobile interface. Now, Twitter has acquired popular iPhone/desktop app Tweetie, and will be turning it into Twitter's official iPhone application.
Tweetie is the most popular app for iPhone and is currently $3 in the App Store. According to a Twitter blog post, in the coming weeks Tweetie will be renamed Twitter, and will be available for free in the App Store. No word yet on an official Twitter app for Android phones, but I'm guessing it can't be far behind.