NSA's BIOS Backdoor a.k.a. God Mode Malware
Here's another reason why using a free BIOS like Coreboot is a great idea - if malware attempts to reflash it with an infected copy of presumably the original proprietary BIOS, it will be very, <i>very</i> obvious.<p>Another "tamper detection" idea, if you're OK with using proprietary BIOS and/or Coreboot isn't supported, is to add a small piece of code to the existing BIOS that simply hashes the ROM contents and prints out a human-readable string to the boot screen based on that. If it goes missing or changes, then you can suspect something happened. Of course, the trick is to <i>not</i> have everyone doing this with the same code, or backdoors will just detect and spoof it. I modded the BIOS of an old machine I have to do this, it prints out "Customised by {my handle}".