3D graphics use a lot of render power of that there is no doubt. The trade off for Sony is to limit developers to 720p resolution in their 3D games while the console can render 1080p images. Sony say the limit has been imposed as 1080p 3D image would not render smoothly at 60 frames-per-second on their flagship console.The company stated that humble gamers would NOT notice the difference in resolution and only trailed engineers would.
So, you gamers are not trained to spot the resolution drop - why bother having 1080p at all then?
Or maybe theirs so much other 3D stuff going on your eyes get confused or are Sony getting confused?