I've read countless posts about resetting the PRAM and replacing the battery, none of which worked. Disgusted and ready to chuck it I tried powering it up one more time and to my surprise it booted fine -- I had left the keyboard unplugged from the motherboard! Any attempt to reconnect the keyboard and boot resulted in the failure to boot. However, I have an external USB keyboard plugged in as I type this, and all is working fine!
Hope this helps somebody out there...

