Where are the diagnostic LEDs located on a 27" iMac (late 2009, 3.06GHz)?

My iMac stopped booting. The fans and hard-drive spin up but the screen stays black and I can hear that the hard drive isn't been read after the initial spin up.

I decided to open it up to take a look at the diagnostic LEDs to see if it might help me diagnose the problem (the most obvious failure is the logic board). I can't find the LEDs anywhere, where are they? Does my iMac even have diagnostic LEDs?

The other possibility is that I can't find the lights because they aren't lighting up, what would this suggest? The fans, the hard drive and the CD drive all spin up fine so at least I know that there is some power running though it.