(Did I throw out the receipt? Whatever the reason it might have been, there goes $20.)
I've noticed that setting the gamma to be one big notch lower in Windows from the default middle setting seems to make it match 2.2 in OS X's Display Calibration Assistant.
I think the very top is 1.0; the middle is 1.8; and the "click any part of the slider below the selector once" be 2.2. (Clicking it twice gave me crushed blacks on the wallpaper, unlike how it looked in OS X.)