Microsoft officials seek to assuage concerns that its implementation of UEFI in Windows 8 will prevent users from loading non-Microsoft operating systems or applications on their machines. The boot sequence for Windows 8, which is due in 2012, will be markedly different from that of its predecessors. The most notable difference is that the firmware will only load code that is signed and authenticated by a key that s embedded in the PC hardware. The goal of this is to prevent malware such as rootkits and bootkits from staying resident on machines and reloading each time the machine is restarted.
Source: https://threatpost.com/microsoft-defends-secure-boot-windows-8-092311/75683/