UPDATE: A bit of clarification. There are two games: Just Dance Now and Just Dance 2015. Just Dance Now is the free app and, as long as you have a computer screen or a smart TV nearby, it's all you need to play. This is the makes-your-mobile-the-controller game.
The other game is Just Dance 2015 which doesn't do this. It's the traditional Just Dance experience that works with Move, Wiimote or Kinect controls.
ORIGINAL STORY: Just Dance Now, just like the new SingStar, will use your smartphone as the controller.
You download a free app, press a big simple button and, shazam!, you're in. It doesn't matter if you connect via 3G or 4G or Wi-Fi (or at least that's what executive producer Jason Altman said) - everything connects.
What's more, a whole pack of you can dance along. He suggested even entire audiences at concerts could get involved. He demonstrated this by inviting a pack of dancers onto the stage to dance along to a song.
Some girl called Alberta, positioned up on the balcony, apparently got the highest score, although it all looked a bit scripted. Nevertheless, your phone could serve you a personal score and it all be compiled by the host console game.
Altman described it as "latency-free multi-device online gaming".
Meanwhile, Just Dance 2015, due this October on a host of platforms (Xbox One, Xbox 360, PlayStation 4, PlayStation 3 and Wii U) features something called Community Remix. Here, the community do their own dances and upload them, and the community dances along to them. It's not clear if you'll have any video editing options, although it seemed, from a brief glimpse, you may.
Elsewhere, omnipresent music mogul Pharrell Williams will be in the game, as will Ellie Goulding and Calvin Harris.