EFI configuration or how to destroy your iMac Intel

Last week nak.journalspace put online step-by-step instructions for accessing the EFI of an iMac Intel.
We didn't write about it, because one of the big downsides of this method is to have to boot from an external source - CD or hard-drive - in order to boot afterwards. According to Ars Technica, it seems we were well inspired not to. Indeed many of those who tried to change EFI's settings came out with a permanently unbootable computer that no reset on earth could revive.
Be careful then, and let bold experienced hackers open the way first.
[Update] A way to reinitialize EFI and the firmware has since been found. All you have to do is to unplug the power cord while simultaneously pressing and holding the power button on the back of the computer. However we still recommand the greatest caution when dealing with EFI settings.