> Worryingly, this was enabled by default on my MacBook Air as soon as I upgraded from the copy of Lion it shipped with. I did not enable this feature.
So it looks like you don't have to enable it yourself manually. I can neither confirm nor deny this myself, just going off what the author has said.
In my case, I haven't enabled the "Reset via AppleID" feature, upgraded from Lion to Mountain Lion, and when I get the "Forgot password?" popup on the login screen, the only option to reset my password presented is to use my "master password", no mention of using Apple ID.
I did notice an unrelated bug, though: as can be seen in the screenshots, the upgrade appears to have enabled Guest User login.