I just had a fun day.
I’ve been playing around with the preview release of Mountain Lion and for some reason decided to do a remote lock of my Mac via iCloud.
The result was a *VERY* locked Mac. Even when entering the correct password on the EFI boot page the Mac refuses to boot. This is because it sets a new variable in the nvram, which Lion and Mountain Lion detect and lock up upon detecting.
The fix is to boot into a copy of Snow Leopard by holding in Option on boot (I used a copy on SD card) and run the following in terminal:
sudo nvram -c
Hopefully this will return your Mac to a functional state. You’ll need to re-enable Find my Mac in System Preferences, and remove the EFI password through the recovery partition.