Today in the "cool iPhone apps you'll download and then show to your friends once but never actually use, not really" file is Mosaic, a very cool little tech demo you'll download and then show to your friends once but never actually use, not really. It's a way to connect a bunch of iPhones together so they show the same image, stretched out--rasterbated, if you will--over all of the screens. It actually seems to work pretty well; there's a lag when moving them around, but the method to connect phones is seamless and intuitive. Just place the phones in whatever configuration you want, open the app on all of them, and swipe from one to the other without pausing.
Multi-screen displays have visibility problems; we like the idea of them a lot, but in practice, gadgets like the Toshiba Libretto W105 suffered from the division between the screens. The insurmountable black border between screens makes them not really ideal for viewing...well, anything, really. But it's definitely a cool demo, and looks like other people thought so too--it won "best mobile app" at the PennApps 2013 hackathon. Read more about it here.