UberTwitter beta arrives on the iPhone, iPad and iPod touch
UberTwitter, other than being a very polished app, only really has one unique feature: UberView (please forgive my continuing lack of umlauts). UberView, with a quick flick of the phone into landscape orientation, shows you a preview of links contained in tweets -- handy for when you don't want to open up a browser, or indeed expose your iDevice to potentially malicious websites.
Other than that, UberTwitter for iOS provides the usual slew of Twitter miscellany: tweet pictures, add your location, change your avatar, send Direct Messages (!!!) and more.
UberTwitter for iOS [iTunes] -- Free