Here's a side-by-side-by-side of the Nokia Lumia 920's PureView camera up against the iPhone 4S and Galaxy S III. It's pretty darn impressive.
This video comparison (taken with a Canon EOS 600D) shows low-light shots taken at the same time by all three phones in a dark box that was set up at the Lumia event in New York today. Please note that the Galaxy S III representation isn't fair since my screen brightness was turned down to save battery. But even just comparing the Lumia to the iPhone 4S demonstrates a really awesome showing from the Lumia 920. The Nokia reps couldn't tell us what the ISO setting was for the Lumia's camera, since the firmware and processing isn't final, but it looks pretty amazing already.