I use an ATI Radeon X850 XT video card.
When I try to use theater mode with media player classic, the video being played doesn't show up as full-screen on my secondary display as it should. But this feature works fine with MS Windows Media Player 9.
I am running Windows 2000 Pro SP4 if that helps.
(In case you don't run dual-display with an ATI vid card, theater mode automatically shows videos which are being played in a window on the main display as fullscreen on the secondary display. However, this feature does not work with MPC, but it works with every other media player I have.)
If somebody knows how to make this work, or can write a patch to fix the problem, I would be forever grateful.