Just to throw a much less technical option out there...
Have you checked the diagnostic LEDs on the logic board? Upon removing the [guide|885|access door] and [guide|897|front bezel] and peeling up the black EMI shield, there are three small LEDs above and to the right of the RAM slot.
The first indicates that the machine is receiving power at all.
The second (when pushing the power button) indicates whether or not the logic board is receiving power from the power supply.
The third indicates whether or not the display is communicating with the logic board.
Does the second LED light when the iMac is plugged in and the power button is pushed?