With all of the various options available, like Echofon, Twitterific, the official Twitter app and everything else, you already have a choice of ways to browse and post to Twitter on your iPhone. But just in case you happen to use Mobile Safari to do it, Twitter has updated its mobile web app, tuning it up a bit to match both the standard official Twitter interface and the "new Twitter experience" seen on the standard website.
As you can see at right, the bar that's normally across the bottom now sits on top, and you can browse through replies, mentions, and check out trending lists. You can even view some media, like pictures and videos, right there in the feed.
Pretty groovy. Again, I can't think of a reason why you'd use this over a dedicated app on the iPhone, but it's up to you. The new version of Twitter's mobile web app is rolling out to "a small percentage of users" this week, and more later on.