Well, I checked my machines and none of them have the update. They run all the time; have been constantly attached to the internet during the suspected time period; and they also have update turned off. So if M$ was actually doing this, I would have expected to see it...
I find the concept of uploading unauthorized content to be reprehensible, to the extreme and I would expect others to feel the same way. Then there are the liability issues, if the update did harm to some. I would think that the liability issues alone would effectively prevent M$ from such an action.
There there is the potential, that if M$ can do it, then there is nothing really stopping some lone hacker from doing the same other than the secret of how and secrets are notoriously hard to keep. All it would really take is for someone to be recording the network traffic as it was happening to them. Whatever key used to access the system could undoubtedly be decoded and then used, with some DNS spoofing (so the phone home signal goes to the hacker rather than M$), in a less than noble cause which in the end could potentially bring Microsoft to their knees. Even Microsoft could not withstand the public uproar if they were implicated in such an attack.