The iMack bongs nicely, and the f14/f15 keys make the screen brighter and dimmer, but the screen stays black throughout the boot process. I do not have an external display to check to see if that works. It has a bit of hard drive noise and it will make keyboard noises if you hold down a key for a long time so it seems to be booting fully, just no screen display.

I looked at http://support.apple.com/kb/TS2094 but that seems to be only for the iMac G5 iSight 20", not the 17" - in any case resetting the PRAM did not help. I opened up the case, removing the front bezzel and peeling back the black metal tape, but cannot find the diagnostic LEDs - where are they?