Adobe Principal Scientist Lars Borg:
Macintosh, in 1984, introduced us to desktop publishing and to displays with shades of grays. Publishing at that time meant printing presses, and the dot gain of a typical press (then and now) corresponds to a gamma of 1.8. As color management was non-existent at the time (the first color management solutions did not appear until early 1990s, when color displays became more available), Apple’s pick of a 1.8 display gamma enabled the Macintosh displays to match the press.
Snow Leopard uses a new default gamma of 2.2, resulting in a darker, deeper shade of color in your display. I have been calibrating my display (check out SuperCal) with this setting since Mac OS X Tiger to match my local printer’s Windows dominated operating environment, and it’s no surprise that Apple is going down this road, as it’s only logical & easier for users like me to deal with production.