I'm going to try 'tobyglyn's' suggestion first because it makes sense. My theory is that the initial surge of being plugged in with the power with the power button being pressed at the right time is getting the power supply over some hump.

There is something about catching the power supply at a good moment just after it getting power from the AC line and pressing the power button that does the trick. Simply reseting the SMC does not have the same effect. I can get the system to start up by unplugging it and plugging it in several times repeatedly and eventually pressing the power button will start the system. I VERY SERIOUSLY do not think it is the processor board. The PWR SUPLY test LED is good at every step of the process with the exception of getting fans and power when I reinsert the processor board. system and when though every isolation step and never got anything definitive. I'm Apple Certified so I tore it down to min. Pressing the button again generally does nothing. I went to turn it on this weekend and there is a small initial click followed shortly by a second and final slightly different sounding click and then nothing. My MP 2009 remains off most of the time since it is just a backup.

I have two towers a MacPro Early 2008 (my production system) and a MacPro Early 2009 (my backup system). I'm having the EXACT same problem Kolvir was having.

Which mac pro power supply