Birdfeed, a challenger to the throne of iPhone Twitter clients currently held by Tweetie, might have the edge in some ways. It caches Tweets, so you can read your feeds even without wireless coverage, and looks super slick to boot.
Among the changes of Tweetie 1.3 are a dark theme, various UI tweaks, Tweetshrink integration, full-size profile image viewing (yay), and a bunch of bugfixes. Grab it now if you like using the best Twitter app available for the iPhone.