From the Free Software Foundation's website:
"Microsoft has announced that if computer makers wish to distribute machines with the Windows 8 compatibility logo, they will have to implement a measure called "Secure Boot." However, it is currently up for grabs whether this technology will live up to its name, or will instead earn the name Restricted Boot."
"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, we are better off calling the technology Restricted Boot, since such a requirement would be a disastrous restriction on computer users and not a security feature at all."
More, and a petition, here: