David Smith has posted an excellent comparison of the iPad 3 screen when stacked up against the various resolutions of existing iOS devices, Apple notebook machines and desktops. He makes an good point when talking about the insanely high resolution of a retina display on a 9.7" screen:
This will present problems for developers and designers of iPad apps unless Apple also releases a new display with either a higher resolution or a HiDPI mode. Otherwise we will no longer be able to view 1:1 mockups or run the simulator at full size without clipping part of the view. Check out the comparison chart he made on his site.