Activision is sticking with what works, and once again using Steam for Call of Duty on the PC.
If you're planning on playing Modern Warfare 3 on the PC, you're going to be doing so using Valve's Steamworks framework. Modern Warfare 3 will be the latest in an ever-growing list of Call of Duty titles to use the Steamworks system.
Activision has previously used Steamworks for Black Ops and Modern Warfare 2. Modern Warfare 3 will use Steamworks for both the digital versions of the game, and the physical versions. PC owners who don't mind downloading their games can pre-purchase Modern Warfare 3 on Steam now, and have it ready to play the second it's released later this year.
Steamworks is a suite of tools created by Valve that give publishers and developers access to the various elements of the Steam platform. Through Steamworks, publishers can implement achievements, use Steam's ready-made voice chat and anti-cheating systems, and gain access to Steam's various backend systems that make authenticating and protecting content relatively easy. Steamworks also has the advantage of being free, which is undoubtedly a point in its favor.
Call of Duty: Modern Warfare 3 comes out for PC, PS3, and Xbox 360 on November 8th.