John Gruber of Daring Fireball on Monday pointed to a series of Twitter posts (1, 2) outing UI resources that scale to larger sizes in the latest release of Mac OS X Lion, which arrived last week.
The new elements include the pointing-finger cursor in Safari, the "grabby hand" in Mail, and the camera cursor for taking screenshots and a few others. One straightforward reason for the change could be that Apple wanted to improve the look of the Universal Access zoom feature. But, reports from some Mac Mini users outputting to HDTVs over HDMI that upgrading to 10.7.3 caused their system to reboot into HiDPI mode have added to the mounting evidence that Apple is planning for high-definition Mac displays.
Apple added HiDPI modes to Mac OS X Lion last year, but they were previously only accessible by installing Xcode. HiDPI is modeled after the UI resolution doubling that takes place on Retina Display iPhones.