The official Twitter for iPhone app has updated, and with the latest update it became a universal app, meaning it has native iPhone and iPad versions. While the iPhone version continues to incrementally improve, it's the iPad version that is really remarkable.
It took me a little while to get used to it, because the user interface is fairly busy. But you get a heck of a lot of bang for your buck with all that busy-ness. While most Twitter apps on the iPad work best in portrait mode, and Twitter for iPad works fine that way, it really seems optimized for use in landscape mode.