UPDATE: TweetDeck for iPhone is now LIVE in the iPhone App Store. Get it here, and then tell us what you think in the comments.

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TweetDeck, the leading desktop app for Twitter, is set to launch its iPhone version tonight at 9pm Pacific Time, the company has informed Mashable. The app will be available for free, and has already been submitted to the App Store, the company informed us. While the announcement will take place at midnight ET (9pm PT), the availability of the app is dependent on how long it takes Apple to approve it.

News of the app, but not its launch date or price, was leaked by Digg founder Kevin Rose on his blog today.

We've previewed the upcoming iPhone application at Mashable and can confirm that it's quite stunning, bringing TweetDeck's beloved columns and gorgeous interface to the small screen. The app also syncs with the desktop version, providing a complete desktop and mobile solution for Twitter power users. The feature list includes:

- Column based friend groups (like TweetDeck desktop) - TweetDeck desktop syncing (make a change on the iphone and it's reflected back on the PC) - Facebook toolbar like status updates - Multiple account support - Flicking to move between columns - Shake to refresh

The launch will mean that TweetDeck competes on two fronts: against Seesmic Desktop on the desktop, and popular mobile client Tweetie on the iPhone. What TweetDeck brings to the table is a holistic experience: leave your desk and have the exact same experience and data on your phone. Many Twitter applications are moving in this direction: Tweetie already has a Mac version, while an iPhone app is on the way from Seesmic.

The trick for TweetDeck will be to leverage its desktop dominance to win in the iPhone market, too. Our previews today suggest it has a fighting chance.