My EFI was version 1.7 and I downgraded it to 1.6 by following the instructions in the answer given on How to downgrade EFI firmware back to version 1.6. (I made it to the end of step 4 and heard the "angry beep".)
Now when I power on the laptop the sleep LED lights up, fade to a dim solid light for a few seconds then does several (I think eight) quick flashes before returning to a dim solid light, and then this pattern cycles continually. The screen is off, the keyboard is unresponsive (no caps lock light), the machine makes no beeps or chord sounds and the behaviour is identical with or without the hard drive and/or memory.
(This question is meant as a DIY resource for others that experience something similar in the future and so responses along the lines of "seek AppleCare" or "it can't be fixed" are discouraged. )