What you're seeing here is Ironfall. The footage isn't strictly a game. It's a tech demo showcasing the kind of effects you'll hopefully see in it when it's done. But for now, let's just enjoy it and hope they can actually pull this off.
The Gears of War influence is being worn on Ironfall's sleeves here, and while that could be disappointing, you could always view it as the 3DS' version of NOVA, itself a clone of another sci-fi shooter (Halo) which was still useful as an example of what a handheld gaming platform can do.
Below is the tech demo in full. You might want to go full-screen to see it properly.
Ironfall is coming to the Nintendo 3DS e-shop. The game's site is here.