What more can be said at this point, really? It's BioShock Infinite, and not only is it a proper demo, it's the same one that caused the press to shower it with all those Game of the Show awards. So what are you waiting for? I've embedded it after the break. Go watch it!
(Okay, okay, you want more info first? Here's an interesting little fact: this demo seems to confirm that the ten-minute demo we saw back in September was a proof-of-concept level, since Booker gets the same Vigour he did in the old demo, Murder of Crows, in a completely different location.)