Original post by: Dan ,
It sounds like the firmware boot loader is corrupted. I've seen this only once before. We suspected some malware running under Windows messed up the firmware. Apple has addressed this risk in the most current firmware. Sadly, you need a bootable system to apply the update (catch 22 here). I would visit an Apple Store to see if they can fix this (they should do it for free). Otherwise, it will likely need a new logic board - Ouch!