At the bottom you've got a settings button and a new tweet button that slides the entire interface down to reveal a notepad-styled compose window -- another interface idea we think others will pick up on quickly. As with any good Twitter client, you can attach photos, geotag your tweet, and shrink URLS.
These basic interface elements are the same regardless of portrait or landscape orientation, although there's a massive empty space to the right of timeline in landscape view when you first open the app. But wait: selecting a tweet brings up a slideable sheet that fills the space with a number of context-sensitive panels -- user profiles, a browser pane that loads the links in a tweet, a pinch-and-zoom photo viewer. All of the panels can be slid back and forth so you can see what's underneath and switch between them -- it's almost like overlapping windows. There's no really good way to explain this, so just check it out on video:
Twitter's sliding panels UI is definitely one of the most unique and usable iPad interface concepts we've ever seen, and we're pretty sure it'll be aped by all sort of other apps in the near future -- and just like pull-to-refresh, there are going to be good implementations and complete catastrophes. Indeed, even Twitter's implementation can get a little cluttered and chaotic. But it's nice to see Twitter and Loren Brichter have carried on Tweetie's legacy of UI innovation with Twitter for iPad -- and that they're doing it in a great free app makes it even better. We're told it should start appearing in the App Store now, so go grab it.