The new system allows a user to link their Steam account to as many computers as they please.
How it works is simple, it doesn't install anything extra onto your computer, first you just participate in the Steam Beta that's out right now which means it's completely optional. Once signed up for the beta, it's automatically set.
Steam will check every time you log on what computer your on without using any malware. It will automatically trust whatever computer you started the beta on. From then on if someone tries to sign on with your information, before they are allowed on, they will be required to input a special onetime use code which can only be acquired through an email which is sent to the Verified Email address linked with that Steam account. Unless they have that code, they can't get into your Steam Account even if they have your username and password.
This will also help if you want to add a new computer to the trusted list. You log on to your steam on the new computer, go to your email, select whether or not you want to trust that computer, and your done, and if some hacker tries to you can just hit deny and they will not be able to get on.
And you'll be able to set up a kind of, "Master Control" computer, scary name but just listen. It'll essentially be the only computer that can change major account details, like Password, email and what not.
So no worrying about "Trusted Computing" crap, Steam isn't installing anything new on your computer, it's just adding some extra checks when you log on. So n o need to worry about Hal saying, "I'm sorry Dave, I can't let you log onto your Steam Account." Or some stupid program that will let outside companies control your computer.
This is not Steam or Valve trying to control how you play games or how much content you can or can not access, this is just a long needed, SECURITY update for the user that will help stop a lot of phishing scams and help alleviate some of the stress on Steams User Support since their main flow of support tickets comes from people who've had their accounts hijacked.
Gabe Newell has so much confidence in this new feature, he gave out his usename and password for his Steam Account to the public. And nope, he hasn't been successfully hacked yet.