They do exist at least in concept form:
http://en.wikipedia.org/wiki/BIOS#Virus_attacks
And Mebromi was spotted in the wild:
http://www.net-security.org/malware_news.php?id=2143
Thing is, since a BIOS is specific and since flashing a BIOS is complex, I doubt these kind of viruses are common (for now). I think Award has a hook which the virus uses but in the future there might be a virus which first reads the current BIOS, modifies it and writes it back out.
AFAIK the current ones just overwrite the BIOS with their malware version so unless the motherboard is the same they'll most likely brick the system.