It's probably fair to say that Twitter clients were the first "killer app" category for the iPhone -- Twitterific lead the charge way back when, but Tweetie came quickly after and then the flood gates were opened. But now, a late and unexpected challenger has entered the ring: Twitter themselves. Mashable has the screenshots -- Twitter has always had a slimmed-down mobile client (and it's still up right now at m.twitter.com, while the new preview is at mobile.twitter.com), but this one's a little shinier, with almost all the features and graphics of the main site.
A few things didn't make it -- you can't skin your page as you can in the browser client (although none of the third-party clients that I know allow you to do that from the mobile client either). And lists are missing as well, though perhaps that's because Twitter doesn't quite consider them ready for prime time yet. Still, it's a definite improvement from the other mobile version, and the official blog on the subject says that they still have lots of visitors, despite all the other third-party clients out there.