qStatus is the App Store subset of the qTweeter Jailbreak app, and Echofon is the new name for Twitterfon. Confused yet? Don't be, we've got the basics on both these iPhone and iPod touch Twitter clients!
qStatus [$0.99 - iTunes link] is designed to quickly let you update your Twitter (and/or Facebook status), either with text, or by sharing your current song, a photo from the camera or library, or a video from iPhone 3GS. There's no reading functionality, but there is the typical Gx5 attention to awesome interface detail. Unlike qTweeter for Jailbreak, of course, there's no multitasking "swipe-down-to-tweet-from-anywhere" functionality either.
Echofon [Free - iTunes link] or Pro [$4.99 - iTunes link] is, as mentioned, the new name for Twitterfon and the new Pro version includes Push Notifications (with optional "sleep" period so it doesn't bother you , sync with Echofon for Firefox on the desktop (formerly Twitterfox). Non-Pro users get bug fixes and the name change.
If you get your tweet on with either, let us know how they work for you (or help me test them via @reneritchie).