If you go to iTunes US and UK Tweetie no longer shows up. Links to Tweetie 2 on itunes also now return error messages. However according to Mobile Entertainment site people who own Tweetie 2 can see an icon to download the now-renamed Twitter app as an update in the desktop version of iTunes, but the store is not actually allowing it to be downloaded yet.
The 2.4MB download will presumably be available imminently and will be free – a promise Facebook made on their blog entry announcing the Atebits purchase back in April.
The iPhone app will join the very successful Android 2.1 app launched April 30 http://blog.twitter.com/2010/04/twitter-for-android-robots-like-to.html