To provide a unified experience and offer arguably one of the best twitter clients, Tweetie for iPhone was bought by Twitter. If you update your Tweetie app today, it's now free, and has a few new features.
First, it's no longer called 'Tweetie'. The new name is 'Twitter', which is exactly what the folks at Twitter want. If you search for 'Twitter' in the App store, you will now see this 'Official' twitter client.
They added a few new feature like browsing trends, finding popular users, changed the re-tweet to be just like twitter online, in-app sign-up, and more.
Overall I think the changes are good ones, except for one that sticks out like a sore thumb. The new application icon; it looks like a ghost (to me at least). Here is the official release from the folks over at Twitter.