The latest version of Mac OS X may be phasing in support for higher resolution displays. A source with the second developer preview of 10.8 Mountain Lion has told Ars Technica that double-sized icons are appearing in a few places, including Messages, the updated version of iChat. As shown in the screenshot below, the green audio chat icon is twice as wide as it should be, displaying in 2x resolution.
Ars' source interprets this to mean that a "Retina [MacBook] is close; perhaps concurrent with the release of OS X 10.8," which is certainly a possible explanation given the Retina display on the new iPad. However, this isn't the first time that high-resolution icons have showed up. The current version, Lion, includes double-sized cursors and other assets, so it's also possible that Apple is simply trying to build in better resolution independence over time. The consumer version of Mountain Lion is expected to be ready by this summer.