Sony could "turn it on" as easily as Microsoft could, and I don't see any reason to trust them more. Don't kid yourself; they'd love to eliminate trading and used-game sales. They're not looking out for your best interest, they just didn't think they could get away with it. For all we know they had a similar system in the works (competing companies generally try to match features) and just hadn't revealed it yet. Besides, Microsoft will have to be on their best behavior now. They're trying to play the "you spoke, we listened" card and even a hint of re-implementing their hated features will call down a gigantic tantrum.

I'd say you're as safe with Microsoft as you are with Sony at this point.