I beat the DLC Old World Blues, but have yet to earn the Cardiac Arrest achievement. From the description and everything I've seen online it's one of those "freebie" achievements you get for passing a certain point in the game's plot (as getting the item involved it part of the main quest for the DLC). I also fully upgraded the suit as well so it's not that.
So has anyone had this happen? I know it's not because of something like messing around with the command console, because I still earned the Make up your Mind achievement in the same sitting.