It doesn't have to be the right mouse button -- the standard mouse button works as well. This is the new "rectangle mode" of the white balance eyedropper tool.
Instructions: Click-and-drag with the eyedropper tool to define a rectangular pixel area. Upon releasing the mouse, ACR will use all the pixels within the marked rectangle to set the global WB. The usual single-click WB behavior of the dropper tool is still available and is unchanged.
This feature has been available since ACR 8.2 (and is unchanged in ACR 8.3 RC).