By design, the feature is intended to keep unwanted and potentially malicious software off a system by preventing unauthorized binaries to load during the boot process. However, the FSF believes that this technology could be abused and simply be used to not allow users to load certain free software.
"We are concerned that Microsoft and hardware manufacturers will implement these boot restrictions in a way that will prevent users from booting anything other than Windows," wrote Matt Lee in a post on the FSF website. "In this case, a better name for the technology might be Restricted Boot, since such a requirement would be a disastrous restriction on computer users and not a security feature at all."
Lee suggests that users should keep their ability to decide whether they want to enable or disable boot restrictions and there should be a way that will allow users to install a free OS.
"Computer owners must not be required to seek external authorization to exercise their freedoms," Lee wrote. If Windows 8 will prevent users from installing a free OS, Lee believes the result may be "complicated and risky measures to circumvent the restrictions", and the " popular trend of reviving old hardware with GNU/Linux would come to an end."
It's a good idea to keep an eye on such new features, but I would think that it is rather unlikely that Microsoft will shut out other OS from its Windows 8 platform. If Microsoft was almost broken up over the integration of IE in Windows, it's fairly easy to imagine the potential antitrust effects if it were to shut out other operating systems.