@DeadX07: It seems like it's a pretty solid feature to me. It's a guaranteed way to make sure that a user's services are going to Just Work. I've got five Windows 8 machines, and with each one, there's been almost zero configuration. Just log in and everything's there. Machine dies? Move on to the next and you're back up and running with little down time. I just wish there was a way to batch-install (Not)Metro apps when you move between machines.
@wkempf: A NFC/Bluetooth/WiFi enabled phone app that generates the key would work for me. Say, when your Microsoft account detects a login attempt, it would send a request to your phone and you could either bring the phone within NFC range of your tablet or the phone could wake up and communicate directly via Bluetooth or WiFi, or even just prompt you: "Your credentials are being used to sign in from <Computer Name>, do you wish to allow this? Yes/No".
Maybe multiple levels of prompting. For all events such as login, Store purchases, Wallet transactions or any combination thereof.