I've had plenty of experience with slowly dying motherboards with intermittent POST problems that get worse and worse until they no longer turn on.
I've also had one mobo that died a sudden electrical death and would not show any signs of life at all.

However the current experience is new to me.

P5Q PRO with QX9650 CPU, GTX660GPU, 8GB DDR2 RAM.
Was having intermittent POST/freeze problems, but not so frequently. Generally could work for days and multiple suspend/resume cycles without freezing.
One day system restarted in the middle of work - Not typical behavior; Clue #1?

Never POSTed once since then. Symptoms are like this:
Push power button, the board turns on - fans, lights start, drives get power (blinking optical LEDs, hard drive start-up noises), and nothing.
No POST beep, nothing on screen.

So far this is what I'm used to with dying boards, but here's what different this time:
In this state, the board does not respond to ANYTHING. Not the reset button (which normally triggers a visible response from the optical drive LEDs at least), not the power button, not even a long power button push. The only way to turn it off is to switch off the PSU.

Tried the usual things - remove some/all RAM, change RAM slots, remove/change GPU - no change.
No beep codes that one would expect when started without RAM either.

It is as if the system stays in permanent reset state. The kind of behavior that I would expect if, for example, the reset switch was stuck or the reset pins were shorted. However, this does not appear to be the case (checked the button, the pins, and even started the board outside the case).

Tried a different, known good power supply as well. Same behavior.

I do not have a different compatible CPU at the moment. I might have a PCI POST card somewhere. If I can find it, I'll see if I can get any POST code at all.

Any ideas on what can cause this particular "stuck in reset"-like behavior?