All systems that ship with just OEM and Microsoft, could not boot a standard copy of Linux. Microsoft revealed in a BUILD session on UEFI that the following requirements must be met for Windows 8:
All firmware and software in the boot process must be signed by a trusted Certificate Authority (CA)
Required for Windows 8 client
Does not require a Trusted Platform Module (TPM)
Reduces the likelihood of bootkits, rootkits and ransomware