The Windows engineering team has published several articles about the new boot possibilities for Windows 8.  Here’s an excerpt from the most recent post on the subject:

There have been some comments about how Microsoft implemented secure boot and unfortunately these seemed to synthesize scenarios that are not the case so we are going to use this post as a chance to further describe how UEFI enables secure boot and the options available to PC manufacturers. The most important thing to understand is that we are introducing capabilities that provide a no-compromise approach to security to customers that seek this out while at the same time full and complete control over the PC continues to be available. Tony Mangefeste on our Ecosystem team authored this post. –Steven Sinofsky

0675.Figure-1---Windows-8-Platform-integrity-architecture_406B7F53Quick Summary
  • UEFI allows firmware to implement a security policy
  • Secure boot is a UEFI protocol not a Windows 8 feature
  • UEFI secure boot is part of Windows 8 secured boot architecture
  • Windows 8 utilizes secure boot to ensure that the pre-OS environment is secure
  • Secure boot doesn’t “lock out” operating system loaders, but is a policy that allows firmware to validate authenticity of components
  • OEMs have the ability to customize their firmware to meet the needs of their customers by customizing the level of certificate and policy management on their platform
  • Microsoft does not mandate or control the settings on PC firmware that control or enable secured boot from any operating system other than Windows

See the rest of the information and the entire post @ http://blogs.msdn.com/b/b8/archive/2011/09/22/protecting-the-pre-os-environment-with-uefi.aspx.