I resolved this issue and am posting fix in case anyone is using the Gefen DL-DVI to MDP with the same combniation. The EDID Mode on the device which prior to 10.8 could be set as the Gefen manual instructs (pg 9 of http://www.gefen.com/pdf/gtv-dvidl-2-mdp.pdf) to EDID Mode 2 (Apple 27") needs to be set instead to EDID Mode 3 (EDID Pass-Through) the brightness control returns without even a restart of any of the three devices -- when this is first done my Cinema screen defaulted 1920x1140 (intead of the full-screen 2560x1440 coming through on Mode 2) so this must be reset under Display Prefrences after switching.