Epic Games' Unreal Engine, which powers games like Gears of War, Bioshock, and Infinity Blade among many others, is officially coming to Mac OS X in the September 2011 beta of the Unreal Development Kit (UDK).
The announcement notes that UDK support for OS X is just a "preview" of its OS X support, but when support is finally implemented in full it should help developers target multiple platforms more easily with their releases, as Valve is currently doing with its Source Engine-based games.
Further down in the post is some other news that should be interesting to iOS gamers: the kit enables multi-display support for iOS devices via HDMI and AirPlay, meaning that (with developer support) you can play Unreal-based games on a TV or other display. A niche feature for now, but one with interesting potential implications for the future.
Source: Unreal Development Kit site