I finally got around to resetting CUDA by using that little button, and while I was at it I reset the motherboard by removing the battery and power cycling it. All was fine for a few hours, then whammo, my Mac turned itself on again. Once again, every time I click Shut Down, it turns right back on like I'm hitting Reset.
So far I've reset the PRAM, reset CUDA, as well as the motherboard, plus started off with a fresh OS install to see if that would fix the power management extensions. Nothing worked.
Maybe there's something funny going on with the ADB port or keyboard I'm using since there's a power button on it. I'll do some experimentation and see what happens.