One point of note, if you know the checksum of your bios it is trivial to ensure that it at least is untampered with, however if the attacker is not simply wanting to mess with such a low level item but instead is looking for a "rootkit" style attack your on your own unless intel proffer some code that looks at instructions and machine cycles in some sort of ratio, of course i still own an athlon
This type of issue will be small scale I believe simply because physical access is required to exploit the processor.