Before that I am using "Echofon" for Twitter updates but now I changed to "TweetDeck for iPhone". Because the interface is cool and you can update your status of Facebook too ! And it is free !!!!
Here is the web site of TweetDeck.
In this web site, there is a desktop software called "TweekDeck" for you to manage Twitter, Facebook, MySpace and LinkedIn in one application. It looks good and I will try it !