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 comparison (taken with a T3i) shows low-light shots taken at the same time by all three phones ina dark box that was set up at the Lumia event in New York today. The Galaxy S III representation isn't quite fair since my screen brightness was turned down pretty far to save battery. But even just comparing the Lumia to the iPhone 4S demonstrates a really awesome showing from the Lumia. 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 man, it looks pretty amazing so far.