Tweetie has replaced Twitterrific on the iPhone. The former is faster, and full of features such as the ability to easily follow someone from within the app. Best of all, the "Send" button is positioned away from the keyboard to prevent premature tweeting. The only minor nit so far is that Tweetie doesn't have an option to keep track of read tweets, AFAIK.
P.S. Thanks to @mdowney for the tip on Tweetie. Now we just need a native Twitter desktop client along the lines of TweetDeck sans AIR-induced memory leakage. Having a hard time justifying $1,200 for the 8 GB RAM upgrade kit from Apple...
