The most invisible form of DRM I've seen is Steam. That's because it isn't just DRM, it's a fairly significant service. I can't sell my game, but I can play it on any number of computers, even if I don't have the original disc. I can chat with friends during my game, and every so often, I can play a game for free for a weekend, or give out a guest pass to my friends for a month or two. Given these benefits, and because the only thing it restricts is reselling, I hardly ever think of it as DRM.