The problem:
My newly plugged-in Acer AL707 LCD didn't show POST, BIOS, or even the Windows boot logo. Only after having reached Windows desktop the display would auto-configure itself and produce output other than a flashing “Input not supported” message.

The cause:
In addition to the LCD, I had a television set hooked up to the tv-out of my Radeon 8500. This causes it to automatically drop the vertical sync frequency down to 50 Hz, which is five Hz below what the AL707 is capable of (55 Hz).

The solution:
After disconnecting the tv, during the next boot the vsync went high enough to allow POST and BIOS to show.

I have yet to find out if it's possible to have the tv hooked up so that it won't affect the vertical sync frequency. I suspect not.