Na verdade, ele será compatível, o Windows 8 está sendo planejado para funcionar no UEFI BIOS. De acordo com este site :
"Mike’s words have caused some confusion in Germany, where popular
media outlets such as Heise reported that Windows 8 would only support
UEFI, a new interface between an operating system and platform
firmware, and not BIOS. This would mean that the majority of Windows
customers would not be able to upgrade to Windows 8, considering that
more than 99% of all users worldwide have a BIOS interface and not
UEFI."
E este :
“Topics in this session include: wow a future platform would evolve
with advanced features in firmware using the latest UEFI protocol and
services; [and] technical details of the latest Windows* 8 platform
requirements including UEFI boot and security will be discussed.”
The ‘Microsoft Windows 8 on Intel Architecture’ session “will provide
a glimpse into: the upcoming release of Microsoft’s next operating
system release Windows 8; [and] the work both companies are
undertaking to deliver this new compute experience.”
Até mesmo a AMI (American Megatrend Inc), que já produz o BIOS anunciado o suporte a UEFI para o Windows 8:
"American Megatrends Announces Aptio UEFI BIOS Support For Windows 8,
UEFI Development PC At BUILD Conference"