Justin.tv launched a live video viewing app for the iPhone today, and it’s smooth.

The app does not allow for broadcasting from the phone, but the image quality is impressive (provided the broadcaster has good bandwidth). From my 3G iPhone, there was about a 45 second lag between action and viewing, compared to the 15 second lag I get when viewing online with an ethernet connection. The app also allows for chat, which updates in real-time and is simultaneous with the online connection.

Live-streaming apps aren’t new. Ustream launched one over a year ago, though that is not currently available in the U.S. App Store (it’s not clear when it became unavailable and I haven’t seen any news about it being pulled. I’ve reached out to Ustream for comment. Qik and Kyte live video viewing apps are also conspicuously absent). Update: Ustream's viewing app is back up in the app store. Marketing rep Shari Folds tells us simply that "We heard it was down for a period of time but it's back up."

Justin.tv VPs Evan Solomon and Caleb Elston said the app will automatically switch between a 200kb/sec stream and a 64 kb/sec stream depending on the bandwidth available (this, they think, is the only live streaming mobile app that does this). The pair also said they are working on new features, including push notifications to tell users when their friends have started broadcasting—this function will be submitted tomorrow morning to the app store—and a categories filter. They also said the app would be coming to Android soon.