The Free Software Foundation voices its worries about Microsoft's new Secure Boot technology in Windows 8, you can read it over here.
When done correctly, "Secure Boot" is designed to protect against malware by preventing computers from loading unauthorized binary programs when booting. In practice, this means that computers implementing it won't boot unauthorized operating systems -- including initially authorized systems that have been modified without being re-approved.
This could be a feature deserving of the name, as long as the user is able to authorize the programs she wants to use, so she can run free software written and modified by herself or people she trusts. However, 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. 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.