Like the iPhone 4's Retina Display, this means that user interface elements will remain the same size, but everything will be twice the resolution and therefore twice as detailed. For example, instead of a 1440x900 pixel 15" MacBook Pro, you could have a 2880x1800 pixel 15" MacBook Pro. All the screen elements would be the same physical size as elements on the 1440x900 display, but with a much higher level of detail.
So developers would just need to develop images with 2x the resolution, and we'd get the kind of blinding detail we've been used to on the small screen. We're still likely a couple of years away from anything approaching a "retina display" on laptops or desktops. But at least we're closer. [MacRumors]