Apple has slowly been sneaking high-dpi features into OS X since the release of Lion, including a hidden “HiDPI” resolution in System Preferences. A post today by Daring Fireball points out the addition of high-dpi cursors in the recent update to Lion, 10.7.3.
As Gruber mentions, the simple answer is that Apple is catering to Universal Access users who frequently zoom in on the desktop, but it is also very possible that the Cupertino engineers are planning Retina-quality displays on the desktop. The change easiest to notice is the updated camera icon when taking screenshots of application windows (cmd + shift + 4, space), but small changes can be found throughout the system. Retina displays are still too cost prohibitive to be feasible on screens much larger than the iPhone’s, but we expect that they will be commonplace a few years down the road.