I have a similar situation here but maybe different issue.
I have a Macbook Pro 17" 2.6GHz (Early 2008, non-unibody, Mountain Lion 10.8.4) with a dead LED backlid panel. I know the display works because I can still see all Desktop icons but very dark. So I had to mirror the screen onto my TV using the DVI cable.
It was all working fine until the OSX 10.8.5 update, my external display cannot detect any signal. So I use Remote Desktop to check & I realized in System Profiler, nothing is detected under Displays even the built-in display. I made sure I still have the "mirroring" option ON in System Preferences but there is no "mirroring" icon at top right hand corner.
Please help! Thanks