Want live TV on your iPhone? Well, you’ll soon be able to have it. If you live in Japan. Japanese mobile carrier Softbank announced today that it will begin selling a TV tuner add-on for the iPhone, AppleInsider reports.

The add-on, which will also serve as an extended battery pack, will stream live television to the iPhone over Wi-Fi. A special application (still to be made) will pick up the live television signal being taken in by the 1seg external antenna. 1seg is a mobile broadcasting service available in Japan and parts of Brazil, as AppleInsider notes.

From the looks of it in the promotional picture, the still unnamed device will be fairly large — almost as large as the iPhone itself. But since it can transmit the TV signal over Wi-Fi, you can keep it somewhere besides attached to the iPhone itself. The picture shows it in a woman’s purse, but a man could just as easily put it in his jeans (or European carry-all, if you’re Jerry Seinfeld). If you want to use it as a backup battery, however, you’ll need to have it plugged into the iPhone via a USB connector.

Television is one of the features that’s popular on mobile devices in Japan that the iPhone lacks. Another is support for emoji — Japanese clip art icons that are used in emails. That is said to be coming in the upcoming iPhone 2.2 software update. Both developments may help boost sales of the iPhone in Japan, which are thought to be less than great.

In the United States, television isn’t as popular a feature on mobile devices. Certain networks, such as Verizon (through its Vcast feature), offer it, but slower mobile data speeds in the U.S. compared to Asia make the option less enticing. As the carriers here start to think about the next generation networks (4G, WiMax and others), that could change.

I know that if I could watch live sporting events on my iPhone, I’d cancel cable television in a heartbeat.