Install a custom colour profile (for example, mine is here: http://www.mediafire.com/?1a5dzowr5ikww3d), open up Windows' "Color Management" tool, check it is associated to your monitor, navigate to "Advanced" tab, for "Device profile" select the name of your custom one. At the bottom of the window, hit "Change system defaults" and repeat the above steps.
Play ArmA 3 alpha, and notice your screen will be pre-calibration. Alt + Tab and cycle back to ArmA 3, not releasing Alt. You will see we are now using the default, and if we release alt to make ArmA 3 the active window again the calibration will cease to be active.