Using a 27-inch Mid 2010 iMac with a ATI Radeon HD 5750 1GB Graphics Card, 2.93 GHz Intel Core i7, and 8 GB 1333 MHz DDR3 Ram. I'm using a Mini DisplayPort to DVI Adapter with a DVI Dual Link TMDS Cable (DVI-D M/M); I've also tried a Single Link Cable. It is currently running OS X 10.8.2.
I'm trying to use my iMac as a second monitor to my PC. I have a Asus HD6570 2GB Graphics Card in that computer with HDMI out, DVI out, and VGA out. It is currently running Microsoft Windows 7 Professional with a Intel Core i5-2400 CPU @ 3.10 GHz, 4 Core(s). I've tried using the Mini DisplayPort to DVI Adapter with a DVI Dual Link TMDS Cable (DVI-D M/M); I've also tried a Single Link Cable. As well as a Kanex MDPHDMITOS iAdapt Mini DisplayPort to HDMI with 5.1 Channel Digital Audio. I've used the Command+F2; I've tried restarting the computers in use in various orders. I've tried doing it in bootcamp on my iMac. I've tried bringing my iMac down to the same resolution the computer is. I asked an Apple Genuis and he said it was the video cards with no conclusive evidence. I have not yet come across a list of compatible Graphics Cards for Target Display Mode.
I also tried making my MacBook Pro a primary or secondary monitor for the iMac. The MacBook Pro, however, is an earlier model (17-inch Early 2008 with 2.5 GHZ Intel Core 2 Duo, 4 GB 667 MHz DDR2 SDRAM, NVIDIA GeForce 8600M GT 512 MB Graphics Card). The MacBook Pro is running Mac OS X Lion 10.7.5