Trusted computing was a plan a few years ago that would stop your PC from running "unauthorised" software. At the time this was mainly about media and came from the darker days of DRM.
The new proposal seems to be that you PC will only boot signed code. This is just an early engineering proposal about the BIOS but reports suggest that MS could make it compulsory to be turned on for Windows 8. Even if this could be turned off in the BIOS it would make to conversion to Linux (or BSD or even MikeOS) start with the "now go to your BIOS setup" step.
I heard about this from Ross Anderson who is something of a personal hero (his book on Security Engineering is brilliant and readable by anyone who reads Linux Format - don't get bogged down by the maths in the first chapter). See his blog entry below.
http://www.lightbluetouchpaper.org/2011/09/20/trusted-computing-2-0/
He also links to http://mjg59.livejournal.com/138973.html which is pretty informative.