I remember when most motherboards would have a physical BIOS write-protect switch, which would've prevented all software from modifying it. These days it's been eliminated from most if not all, for reasons of cost and "but it'll require the user to open the case to upgrade the BIOS!" -- and apparently, the automatic updating software for some laptops will update the BIOS in the background without warning, which is even more disturbing to me...
Unfortunately an open source BIOS isn't sufficient unless it's coupled with an external BIOS chip reader that can dump a motherboard's ROM and verify its checksum matches the expected checksum of a Coreboot BIOS.
http://www.coreboot.org/
I remember when most motherboards would have a physical BIOS write-protect switch, which would've prevented all software from modifying it. These days it's been eliminated from most if not all, for reasons of cost and "but it'll require the user to open the case to upgrade the BIOS!" -- and apparently, the automatic updating software for some laptops will update the BIOS in the background without warning, which is even more disturbing to me...