All screens go black and PC freezes when I turn off a monitor connected by HDMI. I can tell that it's frozen since NumLock does not toggle any more. Power-cycling the PC brings things back to normal.
Details: I have three Dell U2711 27" monitors connected. Two are connected via dual-link DVI, and one via 100 ft HDMI cable with a built-in signal booster by KanexPro. All three go through a Radeon HD 6xxx graphics card into a PC with redundant 750W power supplies.
All screens are running at 1650x1050, with one DVI-connected screen duplicated to HDMI-connected screen.
Latest BIOS and drivers throughout.
EDIT 100-ft cable has signal booster built-in. I connected input end to the graphics card and output end to the 27" LCD.
All works well with a short 3 ft HDMI cable.