Diagnostic LEDs OK but no power

My customer's iMac is not booting at all. When the power button is depressed, the optical drive makes a sound as it resets the optical part, diagnostic LEDs 1, 2 and 3 light up (the LCD is currently disconnected), and nothing else happens. The hard drive does not spin up, no fans start, nothing. I've tried starting it in Firewire target mode but that doesn't work. I tried resetting the PRAM and the SMC, but no results. The customer took it to the Apple store and they determined that the logic board and power supply must be OK, probably based on the 3 diagnostic LED's and said it was most likely the LCD. I don't buy that. Even with the LCD disconnected, I think it should be booting. Am I wrong about this? When searching the Interwebs, I can't find any pinout voltages for the power supply so I don't know what to test for there. Any ideas based on your experience?