I had that problem with my 8550. I recapped the power supply and logic board... still happened. Turned out it was a malfunctioning keyboard that would power it on after it was powered off. Not sure if that's your issue or not. Also, bbraun gave me this info:

"I don't think I can tell you any more than you already figured out, but the power on pin from ADB goes directly to the cuda, which is what controls the soft power.  There is a pullup resistor on that ADB line between the cuda and adb connector, that causes the line to be 5V idle, and the keyboard grounds the line when the power key is pressed.
It could be that pullup is weak?"