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AT&T announced that this was coming a couple of weeks ago, but the 3G version of the app has just today been approved by Apple and has now appeared in in the App Store.
The app costs $29.99, which is admittedly a little steep for an iPhone app. You'll also have to buy and install Sling Box hardware on your TV so the app will have something to communicate with. The result is the ability to watch every show you can watch at home either live or DVRed absolutely anywhere, though.
We think it might be worth it depending on your situation. For example, if you take a long daily commute on public transit, it might be worthwhile, but if you drive it's probably not since it would obviously be dangerous to try to use it while driving.