Chad's excited. Why? TweekDeck for iPhone has just hit the App Store (iTunes link). Doesn't look to be iPhone 3.0 Push Notification enabled just yet, but the hallmark feature of Twitter Groups and multiple, columnar dashboards (looks similar to tabs in Mobile Safari) are all in place. And you can "shake to refresh".
Best of all? It's free!
If you try it out, let us know how it works for you, and how it compares to your current favorite Twitter client.